Here is an example that disproves your statement:
http://www.humblebundle.com/
Average Windows: $3.79
Average Mac: $6.59
Average Linux: $11.63

This is a pay what you want case, where GNU/Linux users pay almost two times more than Mac users and three times more than Windows users. Also, many (most?) GNU/Linux users paid for a Windows license. And lets not forget that for many Windows users software cost is free, as in illegal copies.

Now, do you have any proof of your "GNU/Linux users won't pay for anything" statement or is it just a gut feeling (and we all know what a gut is full of)?