For me I think that browsing this forum and ebay fairly regularly imparts a sense of what is a reasonable price and what isn't. I find that normally the price reflects the time spent to find something, so when you're buying nice and cheap UNC coins that are easy to find you have an idea that they aren't going to cost you much, whereas when you've spent say, 2 years, looking for one of those hard to get dates, you've got a rough idea from all your searching what you want to pay. Then once you've got it you'll definitely remember what the price was haha. Watching eBay auctions end without bidding is always a good idea, if I know I'm going to be looking for something in particular, I'll add a couple of high grade examples (if they're there) to my watch list, then when I come round to getting one myself I've got a few examples with realised prices. LCS also has a pretty good back catalogue of prices paid which is a good reference, along with various books, Tony Claytons website and regular help from members on here you'll be in the know in no time I'm sure!