I try not to pay attention to values very much, because those seem to go up and down, perhaps depending on how much somebody was willing to pay for a card on on some site.
Instead of referring to money, I sometimes look at how many folks are asking for a card and how many are offering a card for trade. If a particular card has (for example) 83 people wanting it and only six offering it for trade, I think the market is making it clear that is a valuable trade piece. If a card has 120 for trade and only 30 who want it, then it can be Ohtani, Judge or Acuna, and I'm still not valuing it as much.
As for my own duplicates for trade, I'm some folks have noticed that if I have four or five of a particular card to trade, I'm much more motivated to trade it than if I have only one of a card to trade.
I love to trade! Check the top paragraph of my profile to see my general trading desires. If you're uncertain about what trade I might be willing to take, just drop me a note and ask. I'm sure we can work something out.