I'm not one to pay a lot of money for collectors comics, but I do have What If #105 (first appearance of spider girl) and the first printing of Ultimate Spider-man . . . I was just lucky enough to pick those up at my comic store when they were new. Not sure what those are worth these days, I know Ultimate Spider-man #1 was worth some money for a while.
I've also managed to pick up battered copies of (the original series) Amazing Spider-man 18, 22, 28, 35, 41 and 48 all for cheap ($10 or less). Cheap beaten up old Amazing Spider-man's are the best! They're still just as fun to read, and it feels like your holding a piece of history.
I had a what if 105 and sold it. Then regretted selling it, and bought another. Lol.