The iOS game for iPad is very good. In fact, it's too good. I deleted the app because I was spending too much time playing Carcassonne. My productivity in other fields has gone up remarkably since I removed it
As I recall, the base game costs $10 and expansions are either $1.99 or $2.99 each. So far the expansions are the River II, Inns and Cathedrals, and Traders and Builders. They are planning to add the Princess and the Dragon this Christmas and Abbey and Mayor next year.
You can play against the computer (with different levels of computer player skill to choose from), play against friends online, or play a multi-player local game. You can play a game of up to 5 players I think (or is it 6?).
Nick