You can look up recently sold items on eBay to get a general idea of what a card's going for right now. For example, holo Dark Dragonite (unlimited) has been averaging about $30 ungraded while Charizard (Base Set 2 unlimited ) and holo Dark Charizard (unlimited) have been averaging closer to $100 and $90, respectively. Depends on the condition too though, so try to only factor in those in similar condition to your cards.