Part of this depends on how limited the print is (and whether or not Willie Mays signs his name over another players' face
).
Personally, I don't collect either Negro League memorabilia or autographs, but my price point would realistically be $200 for a signed print.
$499 would not be out of the question, though, if this were intended to raise funds and it was signed by the mentioned HOFers.
Good luck with the project and please post any images of the painting you might have!