Honestly, purple rings are only colectible to the COLLECTOR, not the PLAYER. THey aren't legal for organized play and barring the few misprint (FF Professor X, AW Crystal) they are the same as teh U 's from teh set.
In my mind they would be worth LESS than the U's for those reasons.
Others' opinions may very.
have both the Purple-ring and the Silver-ring FF Unique Professor X. They are exactly the same. There is no misprint.
How much can you take, and keep moving forward?
The Green Lantern Corps is DC's version of the US Marine Corps.