On Android, at least, it's free to download and play, with limited turns. You can buy unlimited turns in-app for a one-time payment of $3.99, which also unlocks more maps. Additionally, there are various enhancements and powerups you can buy for coins, which you can either purchase directly in-app or complete various offers to get for "free". I believe the game also rewards you coins for doing well.
Personally, I am turned off by games where your experience and success is influenced by how much real-world money you are willing to spend. Naturally, there are people for whom this is not an issue, and I'm sure it's extremely lucrative to target them. But I want to believe that there is room in the indy gaming market for both approaches.