The S-Bahn station is just a pedestrian overpass across from the terminal building; perhaps a five-minute walk from terminal B at FRA. There are actually two train stations at the airport, one for short-distance trains (S-bahn) and one for longer runs (ICE, etc.). The S-bahn station is the closer of the two and encompasses tracks 1-4.

I'd guesstimate RT fare from FRA to Mainz to be in the 10 Euro range, but I'll defer to someone else who's actually done the trip.

FYI the S-bahn does have a first class section, but don't bother. I mean, really, don't.