The cheapest way to the airport. Nothing listed is quicker.
S-Bahn S8 or S9
S8S9 · S-Bahn Rhein-Main, RMV tariffPickup, hours & luggage
Boards at. Frankfurt Hauptbahnhof, Taunusanlage, Hauptwache or Konstablerwache — any S8 or S9 towards the airport
Airport to Frankfurt is Preisstufe 4: a single is EUR 6.90 (child 6–14, EUR 4.10), a day ticket EUR 13.50. The Gruppentageskarte at the same level is EUR 22.50 for up to five people, valid all day network-wide — children count as full people. Do not buy the EUR 14.80 Frankfurt group day ticket: it does not cover this journey.
Luggage. Racks by the doors; it is a commuter train and full at rush hour.