You don't have to pay 100$ to run a marathon. If you have a pair of running shoes and a normal running belt (for water and a gel) that will do. You just have to put them on and there you go. I've done a couple of "Sunday marathons" (aka "long aerobic run") with two friends. They have costed me ~2$ for a gel.
Equipment wise, you don't need to pay 100$ for shoes. There are many <300$ shoes from the last season (last year model) on discount pretty much all year round for 60$-80$. I burn 3 pairs of shoes a year, and even though I could afford expensive shoes, I pretty much always end up finding the ones I want for 70$ somewhere online.
Running belts price varies (from $10 to "as much as you want to pay"), but they are a one time investment. I bought a nice neopren one once, and it will probably outlive my children. If you are running enough to be able to "just go and run a marathon", chances are that you already have a running belt to carry a mobile phone, keys, id, money, gels, water, etc. They are just insanely useful.
I run around 8 hours / week, and my expenses are around 300$ per year. Some years I've paid a bit more, e.g. got a heart rate monitor for 30$, running ear phones for 80$, headlight ~20$, I don't have a running watch yet.., nice running shorts and shirts (couple of 100$s), etc. I've also gone to some races, but I've never paid more than 15-20$ for them. You can do half-marathons and marathons for that money, and some of them are non-profit, so you could deduce that from your taxes if it was worth your time. Also, most races typically give you running tshirts, gloves, and what not as a "finisher" gift.
Honestly, factoring the time I've spent running during my lifetime, and the money I've spent over the last 10 years, it's by far the cheapest hobby I have.
EDIT: That might be a lie. My absolute cheapest hobby is actually swimming. Swim shorts for 20-30$, swim googles for 5-10$, and an annual membership on a masters team (120$/year for 6h of swimming / week, where I only actually end up using 2-4 h). Swim short and googles need replacing only every couple of years, so swimming costs me around 150$/year for about half or a quarter of the hours I spend running / week.
Swimming and running, compared with my other hobbies, are negligibly cheap. I go snowboarding for 2 weeks a year, and that cost me ~2000$ + maintaining my equipment (I basically end up spending ~300-600$ on snowboarding equipment / year). I go cycling with my gf every now and then but not enough to make a 2000$ road bike + equipment cheap. And well I have a motorbike because I want to and that also costs multiple 1000$s dollars per year, I don't even want to know how much.
So yeah, swimming, running, calisthenics, etc. are damn cheap sports. You don't need to spend 1000$s/year on these. If you are tight on money you can probably manage 20h of fun per week by spending ~300$ per year without issues. Obviously, if you want 1-on-1 crossfit training wearing fashionable clothes wearing a go pro and drinking avocado toast kale smoothies you are going to end up paying a lot, but that's not necessary at all.
Assuming you are not poor. Even the lowest price for shoes, you said is $60 and you go through 3 pairs. When I was poor, I was happy to get my one qair of shoes for the whole year at the $20-$30 range. Getting another pair for any special activity wouldn't make sense unless it was required for work.
Outside of that silly aside, running competitively requires running in competitions, and these range into the couple hundred dollar range per run. This is a tough barrier for someone on the low socio-economic ladder. Sure, a dedicated poor person can make it work, but that barrier is practically nil for someone well off.
Of course you can just go and run. No argument there. Training is "free" if you have the time. But most events cost money, and that is where the endurance activity becomes an endurance sport.
I have no idea how people manage to go through 3 pairs of shoes in one season. Maybe those are some kind of special ultra-lightweight running shoes that are designed to fall apart quickly?
8 years ago I bought random New Balance shoes for $65 (New Balance, model MT573 -- just looked up an exact email from Zappos). I've been using them exclusively as my only "gym" shoes for 7 years: I've ran half-marathon in them 2 years ago, I did weightlifting, crossfit, and tennis in them. I regularly exercise 3+ times a week (sometimes more), so I'd estimate that I've logged 1000-1500 miles (~1500-2250km) in them. Sure, they are a little worn out by now, but they are absolutely good to go. And I'm 200-220lbs (~90-100kg).
But long time ago when I was running in high school I've had some other random cheap shoes [1] that I've used for 4+ years. During that time I've been running regularly 3-4 times a week for 10km @ ~40-45min. In warm season I was running in random cotton shorts and a random oversized cotton t-shirt. In cold season I (everyone) was wearing long johns under the track suit and this was it. My father grew up in Seberia doing cross country skiing in way harsher weather wearing something similar.
So yeah ... in my experience running as a hobby is ridiculously cheap.
[1] This was in early 2000s in Ukraine, everyone was poor here, so I'm pretty sure those shoes couldn't cost more than ~$20, which was still a lot for my family.
Equipment wise, you don't need to pay 100$ for shoes. There are many <300$ shoes from the last season (last year model) on discount pretty much all year round for 60$-80$. I burn 3 pairs of shoes a year, and even though I could afford expensive shoes, I pretty much always end up finding the ones I want for 70$ somewhere online.
Running belts price varies (from $10 to "as much as you want to pay"), but they are a one time investment. I bought a nice neopren one once, and it will probably outlive my children. If you are running enough to be able to "just go and run a marathon", chances are that you already have a running belt to carry a mobile phone, keys, id, money, gels, water, etc. They are just insanely useful.
I run around 8 hours / week, and my expenses are around 300$ per year. Some years I've paid a bit more, e.g. got a heart rate monitor for 30$, running ear phones for 80$, headlight ~20$, I don't have a running watch yet.., nice running shorts and shirts (couple of 100$s), etc. I've also gone to some races, but I've never paid more than 15-20$ for them. You can do half-marathons and marathons for that money, and some of them are non-profit, so you could deduce that from your taxes if it was worth your time. Also, most races typically give you running tshirts, gloves, and what not as a "finisher" gift.
Honestly, factoring the time I've spent running during my lifetime, and the money I've spent over the last 10 years, it's by far the cheapest hobby I have.
EDIT: That might be a lie. My absolute cheapest hobby is actually swimming. Swim shorts for 20-30$, swim googles for 5-10$, and an annual membership on a masters team (120$/year for 6h of swimming / week, where I only actually end up using 2-4 h). Swim short and googles need replacing only every couple of years, so swimming costs me around 150$/year for about half or a quarter of the hours I spend running / week.
Swimming and running, compared with my other hobbies, are negligibly cheap. I go snowboarding for 2 weeks a year, and that cost me ~2000$ + maintaining my equipment (I basically end up spending ~300-600$ on snowboarding equipment / year). I go cycling with my gf every now and then but not enough to make a 2000$ road bike + equipment cheap. And well I have a motorbike because I want to and that also costs multiple 1000$s dollars per year, I don't even want to know how much.
So yeah, swimming, running, calisthenics, etc. are damn cheap sports. You don't need to spend 1000$s/year on these. If you are tight on money you can probably manage 20h of fun per week by spending ~300$ per year without issues. Obviously, if you want 1-on-1 crossfit training wearing fashionable clothes wearing a go pro and drinking avocado toast kale smoothies you are going to end up paying a lot, but that's not necessary at all.