The train station is in the basement of FRA airport , from a quick look fares are v€58- for a 1 hr trip and €42- for the slower 2hr . However check out the special fares and you can have a €39- or €29- fare , BUT no refund and cant change to a later train , there are fares as low as €19- off peak .
I think that the 2hr trains still follow thr Rhein which is great scenery, might be worth checking out with tourist office. If the price is too high why not spend the weekend in Fra-Wiesbaden-Mainz area?
pjq