I've never in my life paid for a background check when buying a firearm....dealers generally don't charge for that. You may be referring to a "transfer fee" if you ordered your gun off the net, or bought private party, and for some reason went to a gunshop to transfer.
Maybe the charge comes with the cost of the gun, but I have bought a few and haven't been charged for background checks. In fact the owners of the gun store even gave us discounts because our son is in the military and my husband is a pilot.
The information pretty much comes from the same sources for any background checks. The end result may be different in whether it is an automated system which just looks for specific parameters or a more detailed one as police departments use.
The difference in whether or not a dealer in any particular state charges a fee for backgrounds or not depends largely on whether that state is using the FBI database check system or a state run system. Here in Pa., we have to use the state run system and there is a charge dealers pay per background check and this is usually passed on to the customer. Unfortunately, too many dealers use this as a money making thing especially for doing private party transfers. The Fed is trying to push all states to use the national system which has the advantage of in states like here, eliminating the fee per call, but there's resistance in some states, like here. Personally, I refuse to charge as much as some dealers do because I don't believe that we should have to charge for this practice at all in the first place. It does become a cost of doing business mainly because the profit margins on new firearms especially is pretty slim unless you are charging full retail price. Then, it's unlikely you'd sell anything at all.
About background checks..........here's a horor story..........
California. $25 for background each time you buy a handgun or long gun. Buy two or more long guns and you can piggyback the background check, 1 check for 2, 3, 4...long guns.
Buy 2 hand guns, it goes like this. Pay for the first handgun, $25 background check and $25 transfer fee. Wait 10 days to pick up 1st gun. To buy 2nd gun you must wait 30 days from date of last sale. Pay for second handgun, pay another $25 for back ground check and $25 for transfer fee, then wait another 10 days to pick up 2nd hand gun. It takes 51 days to buy 2 hand guns!
Buy a hand gun and long gun at the same time, you guessed it, $25 background check for each gun!