Market value of anything is what someone is willing to pay for it. Period.
Use my rule of thumb.
Go to eBay. Do a search for the item you're interested in pricing, but filter the search to look only at "sold listings". From there, check out the most recent sales and what they went for. Sales that are too old may not be representative of current value, but if that's all you have to go on, well...
You won't always find the exact item, particularly if it's not something that hits the market often - that should be a partial clue about the value, since if you can't buy it, that's going to affect the value, right? Make an estimate by finding something that would be comparable in value.