I came to the conclusion that a big bonus for open source won't work. The reason is the open/closed source is a decision, and the bonus would therefore be based on decision instead of the work the team did.

Near the higher places, a 100 points bonus might sound reasonable for having source code. Near the bottom, 100 points means really a lot, I could image it could mean five places higher in the ranking. As I believe judging should primarily be about the game and not about the license, this is too much influence.