Awesome game. In my mind this is the game Castles of Mad King Ludwig should have been
That's interesting. I think of Isle as a much better version of Carcassonne, and hadn't compared it to Castles at all. But of course, the bidding mechanism is similar.
Personally, I see it as a better version of Carcassonne at more than 2 players. For 2 players, I find Carc to be the better game, just because the bidding mechanic has a lot less movement parts with only 2 players.
On-topic, I think ~$30 is pretty much the "normal" sale price for this game now (It was this price on Amazon a couple weeks ago). For anyone on the fence, it's really a fantastic game.