Sedans average 12 to 16 gallons based on size. SUV's can be 20 gallons and may even have an extra tank. Gas in Southern California is going for 4 dollars a gallon for unleaded, so your looking anywhere between 50 and 100 dollars for the gift. Make sure they fill up in the cities not out in the sticks where it might be higher.
Heres a map for Gas prices in the US.
and looking at that map out in the sticks is usually cheaper which is not what I expected.