FBI NICS is a free service paid for by our tax dollars. FBI NICS does not charge dealers to conduct a check.
In those states where a state agency is the POC or "Point Of Contact", there may be a fee charged by the state agency that conducts that check. Tennessee for example, charges dealers $10 for each background check whether successful or not. Often dealers in these states collect a background check fee before contacting the state agency.
Here in Texas, dealers contact FBI NICS directly. The overwhelming majority of dealers charge just a transfer fee and not a separate BG check fee.
If a dealer is selling from his own inventory any cost of doing business should be included in the retail price. IMO, selling a gun from my inventory and also adding a transfer fee and a background check fee is about the silliest thing I could do. Yet there is an FFL who frequents DFW area gun shows that adds a $25 transfer fee on top of his retail prices.
I charge $20 with a $10 discount for LTC holders. That doesn't mean I'm charging a $10 fee for a BG check. I give a discount because I want to encourage every Texan to get their LTC.