Susan,
You put a bunch of numbers on a dart board and throw darts at them and whatever number it hits is the winner.
Seriously, though there is no hard and fast rule of thumb these days but the way many people do it is to try and ascertain the range of prices an item has sold for on either eBay or here at the Collectors Forums along with word of mouth back and forth between individual collectors here as to what they sold an item for or paid for an item.
There is a thread here on the forums that attempts to update this information on the most popular items being sold on ebay at this time.
There is no longer a generally recognized standard to go by. the secondary market is in a continual state of flux- generally right now on the down side.
I know this is not much help but it's about all there is right now.
Dennis