One way from Munich Hbf to Garmisch is €16.90, all on regional trains. A Bayern-Ticket costs €29, less than full fare for two people. It's good on regional trains, which seems to be about all there is on that line, so it would pay to use one. A Bayern-Ticket is also valid for all transit in the Munich metro district, MVV, so you could use it there as well. Note, however, that the Bayern-Ticket is only valid after 9 AM weekdays.
As for the return trip, the first train out of Garmisch is at 6:04, connecting to the S-Bahn in Munich and getting to the airport at 8:17. If that's not early enough you could leave the night before and stay over in Munich or at the airport.