What is the cheapest way io get from Garmich-Parkinkerchin to Salzburg? About how much is it?
By rail, the least expensive way would be with a Bayern-Ticket. That would be valid on all regional trains after 9 AM weekdays, all day weekends. A Bayern-Ticket costs €19 for one person (B-T Single) and €27 for up to 5 people (so €13,50 each for 2, €9 each for 3). The trip takes about 4 hours with a change of trains in Munich.
The least expensive rail ticket from RailEurope was €86 ($54).
Gas cost alone for GaP to Salzburg for a compact car, from ViaMichelin, would be over €20, so renting a car would not be a viable alternative.
And unlike other well-known destinations in the area (Linderhof, Oberamergau, Schwangau, Mittenwald, Innsbruch), Salzburg takes a few hours to reach by car. Echoing Lee, the small amount of time you would save by driving probably wouldn't justify the cost of a rental car over taking the rails.