I'm not sure of the tank size or the current price of fuel BUT there are two options.

Alamo, unlike many other car hire companies, only charge a small premium over the pump price for refilling the tank so it's not a large cost you need to budget for. Also Alamo will now let you return the car with a full tank and then you have no charge.

Just be careful to find out where you can fill the tank near the airport. There is one now famous gas station near Orlando International that was charging nearly $8 a gallon.

Regards Dave