FOSS stands for Free Open Source Software. It’s what Lemmy and everything else of the Fediverse is.
You don’t need to pay for its usage. Just download and set up with responsibility of maintaining being yours and any friends that are interested in being involved.
It’s why I said you don’t need a COOP. Just basic server hardware (like a laptop running a docker.) that has say the XMPP (and a communication system that is FOSS) software and your good to go.
Oversimplified, point is COOP not required