They are worth want someone is willing to pay for them. Are they used? condition? Any issues with he mounting posts. Make sure they are real gmc black letters not red ones painted black.
From GM the list price was about $100/ letter. Fortunately a few people sold oem black letters for much cheaper. I think before yar ran out of black letters they were about 55-60/set and that was a few years ago. I have a NOS set that if I was going to sell I ask for more than 60.
I also have a set of black GMC grill letters (not painted black) in super nice condition. I would be willing to swap outright for anyone with the OEM red reflective letters. Hit me up if interested.