To the uninitiated it can be confusing. If you look up Fantasy Grounds on it's site or Steam you can actually download and play for free. They have some forums (you have to join) you can post where you live, what time you can play and what games your interested in. Then someone who wants to get you as a player will invite you to a game. Their GM will have the ruleset and you can play by invitation for free.
There forums may seem dead but they are quite active. I was invited to 6 games in a couple of days. The problem for me was learning how to use Fantasy Grounds, then learning the new D&D, then learning the 5 other generic sets I was invited to was a big learning curve. I saw some RMC and a RMSS game on the boards. Though they were in different time zones than I. So I didn't get to try them. In my very limited experience Fantasy Grounds is extremely nice for the newest D&D sets. But the quality of the setup drops with the less popular games. The bells and whistles the new D&D had compared to D&D 2 was crazy. The New D&D was set up and click. The others were a whole lot of work.
If you like Fantasy Grounds and want to buy it, they have different tiers. Free you can play with others who have the license. There is a basic license. Then you can buy figurines for your desktop. And if your a GM or your group wants to sponsor your favorite GM you can buy everything for your favorite game or games.
If you want to design modules and host games as a GM you really want to buy everything you can for your particular system. But if your just a player it is more than enjoyable using someone else's license. Unless you just need cool figurines. Then you might want to buy the basic and those
. Essentially you can play for free or you can spend hundreds of dollars. It's all up to you.
If Fantasy Grounds had half the polish for RMSS as the new D&D. And ICE added a character creator app, and the typical bells and whistles, I would be playing RMSS on Fantasy Grounds all the time. It would also be great if ICE set up games from it's forums. That way people could find games and buy products to play in Fantasy Grounds from ICE.