I've only self-pubbed two works so far. (It'll be three in a few hours!) In both cases, I revised the covers based on feedback here. Between the two books, I've sold only 7 or 8 copies (not counting copies I gave to friends and coworkers). So I don't have any data to judge.
But I have one anecdote... Last weekend, I had an idea for a story. Really just a concept, an idea seed if you will, plus a couple of characters. And then for some reason I had an inspiration for a cover. I showed that cover to two friends, and both are now demanding I finish the story so they can read it. I'm rather pleased with the impact of that cover.
UPDATE: I take it back! I've actually sold 13 copies altogether! I lost track somewhere.
"The Mother Anthony" is now up to 8 sales. I can't say that's due to the cover switch, but it sure didn't hurt!
Martin L. Shoemaker
WRITE! WRITE! WRITE! WRITE! WRITE!
SUBMIT! SUBMIT! SUBMIT! SUBMIT! SUBMIT!
REPEAT! REPEAT! REPEAT! REPEAT! REPEAT!
Patience. Patience. Patience. Patience. Patience.