Anecdotal experience but I have found mobile data rates to be extremely cheap in developing countries compared to western ones, even after adjusting for local purchasing power. "Expensive data" isn't really a thing outside of USA, Canada and Europe and maybe a few other countries. In India, for example, the cheapest 4G data plans will give you 50GB+ a month of usage.
I don't think the issue is always about the cost of plans but the fact the network backhaul isn't great in a lot of places. I don't know if it's still the case but India's 4G had wide coverage but terrible speeds and latency (anecdata from Indian friends).
Cost does then come into play when bloated sites eat into small data caps. A 50GB plan seems like a lot if you've got a wired connection at home you use more often. If that 50GB is shared by multiple lines and is the only Internet service you've got it doesn't go that far.
