well basically it is simple, if you pay the 72$ a year membership fee (and I guess maybe you can do it monthly as well)
you have a Free tier paid for allready for 512 meters square of land, which can either be the FREE linden home, which uses up some of the 117 prim allownce but does not use a lot of prims really, because the prims in the linden home are not counted for on a one to one basis, but are discounted... it's like getting a few extra prims and a really nice house in the deal, well that is subject to your interpretation, or you can get a 512 sq meter plot of land on the mainland, which you usually buy from other players, and in either case you would pay no more at all for your land holdings, if you get another 512 plot of land then you would pay an extra 5 dolars a month...and get another 117 prims aparently each additional 512m^2 gives you 117 prims, when you get more land all together, in the same region, you can get bonus prim allownaces for the larger plots, sarting at 4096 you get one extra prim it seems but No worries, you dont have to pay for tier if all you have is the 512 m sq plot, or a linden house, that comes free along with membership...