Well, there is only one version available and that is the "free version"; the way they make money is if you want to buy stuff within Second Life; meaning that you would need to buy Linden dollar - or what the name is. So there are people that "build" stuff within SL; they have to pay to upload "material" such as images and texture to be used when building stuff. If you want to build a house, you need to buy the land and pay "tax" for the land you own.
So in many ways it works out how it would work out in real life; but here is an interesting question for SL fans - when will you take power over the Linden SL puppet "government"? No free elections and no democracy? I thought that the virtual world should be better than real life :-)