The Mousetown app is free. The game is free to play - mostly. But the game can be advanced with the purchase of gems and/or coins using real life money. But then again, this is not necessary. The key is patience.
Early on in the game, you will be introduced to the concept of "rush jobs." It takes time to build any structure. But this time can be skipped with the expenditure of one or more gems.
Throughout the game, you will have the option of speeding up your progress with the use of gems, or playing slowly and using fewer gems. Some gems can be obtained various ways without spending real life money, and should be saved to do certain things that can only be done with gems.