i can't part with the same cash as some of you big city rollers, but i have been fortunate enough to buy 3 covers that were at awesome prices, and i bought them, because I loved them, not because they were cheap, so I got pretty damned lucky.
I picked up the final cover to Midnight Nation (a true grail for me, as it's my favorite all-time limited series) by Gary Frank off of Ebay for less than $600.
(I will post link tomorrow, as the piece is on two pages, and a buddy will do some photoshop magic for me. meanwhile here's the published version.)
My second cover I picked up at from Matthew Clark, because on a lark, he brought it to SDCC2006, just for the hell of it. I got it for $200 bucks.
My last cover is a great cover by Brent Anderson for the second TPB of Rising Stars. the amount of work on this one is unbelievable and i picked it up for $250.