All you need to run a lemmy instance is a bit of time, a computer and an internet connection though, same as running any forum. The costs of all those are rapidly decreasing over time and are well within “eh who cares i’ll just pay it” ranges; a smaller instance can easily be run for less than 5-10 bucks a month on the cloud, or for the cost of electricity if you have an old PC lying around.
A lot of costs of keeping servers running is employing people to do it; if its not too much hassle to keep one maintained, people will keep servers online.
Source: I run datacenters and support various cloud apps