The value will depend very much on exactly which printing you have for any given item and of course what condition it's in.
From the main page, the links in the center will take you to guides to help you figure it out. In this case, you'd end up at the Monster Manual page here:
http://www.acaeum.com/ddindexes/setpages/monster.html
Please understand that in most cases, you'll have items with relatively low value ($5-10), as they are much more common, but you never know - you may have stumbled across some treasures!
If you need more help, you can certainly post here - as TheMilford said, pictures will be the easiest way for us to identify an item.