Really, the only direct cost of lemmy is the development. That's the beauty of lemmy's decentralized nature, the cost of actually running it is spread out among tech hobbyists with spare hardware and time (edit: and only ~$30/year or less for a domain name), or may even have some money to throw at new hardware. For most people, the connectivity doesn't incur any additional cost to whatever they're already paying for internet access.
There are plenty of free and excellent open source projects that neither charge money or generate profits, they're driven by passionate developers who give their and talent for the enjoyment of it and betterment of the community.___