Let's say you plan to run 5 web apps, 3 replicas of each and they all need 4gb of memory.
Fargate demands you reserve 2 vCPUs for 4gb even if your app only needs 20% of 1 CPU, so you're wasting 1.8 CPUs per replica of each app. Often times web apps use a lot more memory than CPU, even with Ruby or Python your web app might be humming along at like 5% CPU load while using potentially gigs of memory.
That leaves you paying:
5 * 3 * 4gb of memory = 60gb of memory = $195.00
5 * 3 * 2 cpus = 30 cpu cores = $886.50
= $1,081.50
You're wasting a ton of CPU here because of AWS' rules on EKS Fargate memory / cpu size combos.
To run your workload on EC2 you can grab yourself (4) m6i.xlarge instances which has 16 CPU cores and 64gb of memory of total capacity which is $560.64 (grand total for 4).
In this case Fargate is double the cost, to me that is a lot. You do have less CPU capacity but it doesn't matter, your apps aren't coming close to that in CPU usage.
If your work load supported it (stateless web apps that can finish a request within 2 minutes) you could use EC2 spot instances too which would be $122.35 total instead of $560.64.
Both regular instance and spot instance prices were taken from AWS' official docs, I just multiplied their hourly values by 730 which is how many hours AWS uses to calculate 1 month.
