That's another thing, I have decided on price ranges for myself, and I won't bid more than that. For instance, for painted plastic guys I only pay about 1/2 price of new. For metal warriors I go with no more than 3$ each, for metal hero's it's about 5$ each. If there's something in an auction I don't want, I won't pay for it, which normally means I won't win as other people want all the figures and are willing to pay for it, but oh well. For nicely basecoated plastic I normally go about 2/3ds of new price, but even nib stuff I won't pay full price for, there's always the (slight) risk of something going wrong, so if I'm going to pay full price I'll just go to GW and get the item right away, and in garuanteed good condition.
Of course I will sometimes pay more for something I really want, or less if it's something I don't want that much. I so think it's pretty important to figure out 'levels' like I have, so you aren't just spending like crazy, and spending all your money on something you might not want, and then not having enough for something you wanted more.
(My best deal was just a couple weeks ago 2 nib (sealed) wood elf boxes for $44 canadian. From GW one box would be $39.50 plus tax (essentially $44), so I got it in the same condition (nib) for half price, I'm pretty happy with that.)