Check out the same items on different websites. If you can get it new from a retailer check out that price too.
Check out finished auctions to see what it sold for. Remember to check postage as most ebayers will put a low starting price on it but a higher postage cost so that there don't have to pay ebay as much.
Finally how much do you want for it. Put it on ata price you are happy with. If it dosn't sell but you still want to get rid of it then lower the price.