Well if one exists for trading actual players (Moss for Portis say) it wouldn't be of much good because it's all how you project them yourself for the coming year(s).
If your talking draft picks there is a very good calc at footballguys.com called the
pick value calculator. It's actually very accurate and takes out the guesswork.
You would still have to look and see if who you covet will still be there or not when your pick rolls around. The calculator can't factor in that because it doesn't have the information to do so.