Yeah, it helps to establish some rules for yourself, too. For example, deciding on your price levels, and sticking to them. Or not bidding on a kit that is currently in production, with some exceptions. For example, if I want to build a Monogram SBD--and who doesn't, on occasion?--I may bid on an auction for one, but I will make sure not to go past the retail price of the kit, which is currently in production. And in the case of a long-lived kit, like that SBD, I might look for older boxings, from when I was a kid, for example, on the theory that the molds were in better shape, so the sprues will tend to be crisper.
I also weigh the rarity or ubiquity of the subject. With model kits, it's one thing. With figures and toy soldiers, it's another. But generally, I like to pay no more than twenty bucks for a model, and less is better; five bucks for a single foot figure; and ten for a mounted figure.
My tolerances are a little higher for Maschinen Krieger subjects, because there are so many currently out-of-print, and they are generally rarer anyway. Same goes for the figures made to go with them.
And in reckoning price, I also try to avoid buying from pirates, if I have a good idea that something is a pirate copy.
But these are just guidelines, and if I've had a beer or two and go on on a Friday night, I've been known to get stupid, too.