Do I have to pay Unity if my game makes a lot of money?

Do I have to pay Unity if my game makes a lot of money?

No.

There is one exception for vanilla Unity and Unity iPhone Basic though:

  • "Unity may not be licensed and used by companies or incorporated entities that had a turnover in excess of US$100,000 in their last fiscal year."
  • "Unity iPhone Publishing Basic may not be licensed and used by companies or incorporated entities that had a turnover in excess of US$100,000 in their last fiscal year."

This basically means that if you are rich and famous, you should get pro / advanced ;)

More information on licensing can be found here: http://unity3d.com/unity/licenses

There are a lot of money-making Flash games out there, but Flash doesn't receive any royalties for those games beyond the initial license for buying the program. It's the same thing with Unity. Any money you make from your game, you keep.