The upper end looks to be about the average monthly developer salary of a poor country with many highly educated programmers (such as Belarus). There will also

The upper end looks to be about the average monthly developer salary of a poor country with many highly educated programmers (such as Belarus). There will also be a pool with talented individuals who are not yet on an average monthly developer salary, and if either your individual project or the scheme as a whole is somewhat successful there will also be a CV benefit in having been sponsored. And of course many developers all over the world would jump at an opportunity to get a month's funding to work on their pet project and keep the IP.