what have you done to contribute to the effort so many others have benefitted you with?

I think this approach will only go so far. Instead focus on how contributing benefits the contributer as well:

• Improving your programming skills.
• Gaining greater understanding of the software.
• Experience to list on your Resume.
• Name recognition.
• The software improves faster and becomes increasingly more usable.
• And many more...

This of course applies mostly to contributing time, not money. Although, in my opinion, time is far more important to open source projects.