How do you make money with open-source software?

You don't, you save money!

(Some companies do make money but many more companies could save money). More companies are concerned with using software than selling software. If users of software fund the open-source development of software then it will make economic sense for the software to be developed open-source.
If 50% of companies that buy commerical database systems instead used an open-source database and spent 25% of the savings promoting and advancing that open-source database then it would all work out well.
The one difficulty is getting the software customers organized to do this. It really helps if someone can jump start the project so that other companies can see the value.

Would you rather spend $100 for for Windows XP or get Ubuntu and spend $25 supporting it?