Some trains penalize you if you buy your ticket on the train. The Heathrow Express charges a 2 pound premium for buying your ticket on the train. You are right - the Gatwick Express did not charge extra when we took it two weeks ago. It is 11 pounds one way. We could pay by credit card or cash on the train.

Rail Europe is charging $19USD which is about 11 pounds, according to xe.net. If you are buying other products from Rail Europe and there are no extra fees, you shouldn't lose any money.

BTW, the regular train may go from the same track so you need to pay attention to announcements.

This trip we stayed one night at the Travel Inn at Gatwick because of our arrival back from Malta. Two pluses - cheaper hotels (we saved 33 pounds by staying one night at the Gatwick Travel Inn instead of the Travel Inn County Hall) and we were able to travel the trains and Underground by daylight.

Frances Toronto, Canada