Hi Steve,
IMHO, it's all down to rarity and quality, and not size or infamy (eg 88's) having just found 10 items that people hunt for a long time to find, their rarity has arguably decreased. Then again, with 6 of them, you can cherry pick what and who to trade them.
The standard 75mm pzgr39 is very easy to find over here. So I would not equate a 2cm Pzgr40 to a 7,5cm Pzgr39.
On the other hand a non Marine (Panther/JadgPanzer IV/70) kwk42 round is a lot harder to find in non relic condition, and probably worth more in excellent condition
That would be my honest opinion, I hope that helps.