Cards generally have no set value, so the best you can do is see what they've been going for lately. What I do is check recently sold listings on eBay and average out the prices from the past month or so. Just ignore graded cards and make sure the condition is similar enough to yours, and that it's the same edition and from the same set. Unlimited edition Shadowless Chansey, for example, has been averaging about $26. Sealed Ancient Mew is averaging about $28 so far this month.