A painting in a book is worth about $2k - $5k these days, but a lot of that value depends on the book. So the value of separate art (not attached to a book, or part of a letter) depends on whether you think it's better in the book or outside the book.
I think he probably made as much art that didn't end up glued into books as did. But that being said, the separate art is far more rare, and that recent sale was a fully realized work (well, as fully realized as Bukowski got), not just a scribble of a man and a bottle and a bird, so...who knows. Ha. It's hard to say because so little separate art sells these days.
But as a rule of thumb, take anything stnickl bids on, cut the winning price in half, and you're at the very high end of what that item is worth. Unless he has competition, in which case he will pay 10 times what something's worth.
Meaning, I think the only man on the face of the earth who would pay $7500 for it, did. Will it be worth that one day? I believe so. But I think it may be when Chelsea Clinton or Hannah Montana is president. Not soon.
I would think for someone who is looking for some Bukowski art to frame and hang on the wall (framing a book ain't easy - or pretty), a few grand would be about what that painting should have gone for. I would pay $3k for that and feel like I got a really decent piece of Bukowski's art and bought something that will appreciate in value.