Although it's been a while, the last time I was there, Budget was getting $6.00 per gallon.
I always fill up my rentals some distance from the airport. Most rentals get about 20 mpg, so about 10 miles away only uses half a gallon... not enough to register below Full on the fuel gauge.
The location of this service station is such that there's not much of a local customer base... just travelers from out of town returning to the airport.
I rented a car in Vegas last year, and the "fine print" on the contract said that if you returned the car with a full tank of gas after filling up, you had to show them a receipt indicating the station was within 10 miles of the airport.
Don't know if that's universal, but there are a lot of hidden fees and taxes around...