There are a number of factors at play. If a poster is extremely rare and has little to no prior selling history, it's impossible to put a firm and accurate "value" of the poster. If there is only one of something, and two people are willing to pay $X to have it, the value is $X even if a thousand other people think that it's only worth $Y. Granted if the loser of the auction doesn't want it any more, and the winner tries to resell it, he may soon find out that the NEW value is only $Y. When you get into these rare piece auctions, prices can soar if there is enough interest and people with deep enough pockets. I recently picked up a very nice poster for a very large amount of money. I personally think I got a great deal on it, since two other ones recently sold for a thousand or two more. But those prior winners might be thinking "crap, my poster is worth less now". It's all subjective, and in the end, only one thing matters = what is the poster worth to you, and how much are you willing (and able) to spend on it.
That covers rare and very valuable posters, but for the more common ones that have a detailed price/sale history, I personally do a lot of research and will usually only bid somewhere around what I feel the "market value" currently is. I use EMP and HA for most of the history, and will usually check out eBay too. The problem with movie posters, imho, is that the market for them is somewhat small. There are significantly more people who collect other items like coins, stamps, etc., so those collectibles have much firmer and controlled prices. With posters, and anything with a smaller collecting community, you can see huge shifts in prices, and there really isn't a "price guide", with BUY and SELL prices, like there are with coins. But even with that in mind, I occasionally will 'over spend' on a certain poster because I really want it, or it's in an unusually good condition. I'm very adamant about condition being good, so will almost always pass up items I feel don't meet my criteria (even if the prices are good). You also need to look at price trends, not just the average. If a poster is seeing more and more demand, the price will steadily (or quickly) increase. Less demand means a steady decrease in price.
Take the Star Wars Bootleg for example:
http://www.emovieposter.com/agallery/search/star%2520wars%2520bootleg/sort/13/archive.htmlThat's a perfect example. Sorting from oldest->newest is the same as highest->lowest price. The conditions are fairly the same, and yet in 8 months, there was a steadily price decline from $249 to $81. What caused this? I imagine that, back in February, one of these hadn't shown up ever on EMP, and a lot of people wanted it. So the price went to $249. That was the, then, "market value". But just 12 days later, the price dropped to $132 on the next sale. Why? Maybe because the prior top bidder had his, and the remaining bidders (outside of the new top bidder) wouldn't go any higher. Less than 2 months later and it's down to $106. Three more months and it's $91. Then two months later, I pick up my copy for $81
. Will the next one sell for even lower? Maybe, but I'm happy with having paid $81, and would probably have gone up to $100 or slightly more. But you can clearly see a trend where not only are more and more of these coming into the market all of a sudden, but the price (and demand) is dropping. If another one doesn't sell for two more years, it might go back up to $249.
To keep myself in check, a couple days before any big auction I will put together a spreadsheet with everything I want (details included), the recent selling prices and trends, what I *feel* the poster is worth, and my max bid. Once in a while I go crazy and go above my "max", but most of the time I walk away and go after something else. And if I see some idiot who is just bidding whatever it takes to win, I won't play those games (unless it's a very rare piece).
But again, it all comes down to you. And your wallet.
