Kelly,
This isn't the prepaid option where you pay for a full tank of gas. It's a new option for Emerald Club members, where they charge you for the gas you used at what is supposed to be local prices. If you bring it back half empty, they only charge you for half a tank. When I was there about a month ago, the station near MK was over $1.70. I filled it myself there because I wasn't sure of the price National charged for this new service, plus I was concerned about how they "estimate" how much gas was used.
I work for a major company and National is our corporate car rental. We've had a similiar deal for years, but it's a set price nationwide, the last time I heard it was $1.65. They didn't want employees trying to hunt for gas stations in unknown areas.