Bowie was selling for $500+ when he was alive. He was very difficult to get for over a decade. I would guess you're looking at $800-900 now for a low end authentic 8x10.
If you want a post card, here's and authentic one on ebay. Hot tip.
Lowest I have seen for authentic "recent" (90's) 8"x10" is about $375 now. With PSA/DNA looking at $500 or so, give or take, still with a fairly recent signature. Nice will be more, so will vintage with mid 70's closing in on nearly double for a decent signature.