Late one night I saw farm expansion in the market for 10,000 COINS (instead of FV money). I bought it and immediately went back to see if I could buy more, but the option was gone. Was that a fluke, or does that offer appear from time to time?
Each of the FarmVille expansion levels can be purchased with coins, instead of FV$; but you have to have reached the required number of neighbors. If you do not have the minimum number of neighbors, you will be required to pay FV$ in order to expand. Each of the expansion sizes also tell you how many neighbors you need.