I would expect them to use the Palm Beach price if there's any difference, because it's "prepaid" and you paid it in Palm Beach. But I've never done it, so it's possible they use the rate at the returning location. There wouldn't be more than 10 or 20 cents difference per gallon, though.
My advice, FWIW, would be not to ever get the prepaid gas. It's a ripoff unless you return the car nearly empty, which is hard to engineer. There is a WaWa a few blocks north of the north entrance to the airport with inexpensive gas, and a Shell station actually on airport property with normal priced gas if you're approaching from the south. It's incredibly easy to gas up before returning the car, and will save you significant money.