No matter if you buy Main- or Privat land, you always have to pay a weekly or monthly tier fee, either to Linden Lab or to the owner of the private estate. The advantage of the private estate owner is that they also accept Linden Dollars in most cases, on the other hand there are special rules to follow on private estate, called "Covenant". You can find the Covenant aggreement of each land in the land info (touch the name of the land on the upper toolbar in the SL viewer).
The value of the land depends on location, view, neighbourhood and ground texture (a waterfront is more worth than grey mountain land). So it depends on your needs which plot is perfect for you.
The minimum size of a land plot is 512qm with a 117 prim maximum (primcount of private island can vary), the maximum size is 65536qm which is a complete sim with a max. primcount of 15000 prims.
512 qm = 117 prims
1024 qm = 234 prims
2048 qm = 468 prims
4096 qm = 936 prims
8192 qm= 1872 prims
16384 qm = 3744 prims
32768 qm = 7488 prims
65536 qm = 15000 prims
Think also about renting land instead of buying. You don´t have to take care of landpricing and have no value risk if you want to move to another place. Also renting is sometimes cheaper because many sim owners have too much space, some offer also additional prims.
There are different ways to search for land. In the SL search click on the "Land Sales" tab, enter the qm size you are looking for and drop down "All Types" to define if you are searching for Mainland or Private Estate.
You can also open the map and enable "Land for Sale" to see yellow colored land which is for sale.
