I have three cosmic-tier base 10 cards. I probably won't end up collecting the entire set. Someone sent me a random trade asking if I would be willing to part with them. I'm tending towards no, just because I generally don't trade anything unless I have dupes. However, this guy has some nice cards in his collection I don't have yet.

I've only been trading a few months - I don't want to get ripped off or offend him with a lowball if I do decide to part with them. The cards are Cable, Loki and Red Skull. I'd be willing to dupe swap for different t10 cosmic cards but they don't have any dupes. Ballpark, what would be a fair trade in return for one or all 3 of these t10 cosmic cards?