Its not only MCO but you'll find the prices much higher by most Rental Car airport locations.

At LAX unless you fill up at the ARCO on the corner of Century by the 405 you'll pay thru the nose at the Texaco alittle less at the Union.

the way around it is as Mary2e put it and I also do is to know how many miles you can drive before the gauge goes below Full and fill it away from any Airport, 9 1/2 times out of 10 you wont have to put any more into the car before returning it and if you have to it'll maybe be a $1 or so, so the damage wont be major.