What would be the easiest/least expensive way to go from Salzburg to Maulthausen for a day trip?
Driving is the easiest way. Mauthausen is about 1 1/2 hours drive from Salzburg. Like Michael says, you can also go by train. You can find train info at http://www.oebb.at/en/index.jsp
we drove from Salzburg; it was easy (something like under 2 hours, as I recall), but do know people who did it by train. If you look at the map of Austria, the closest big city to it is Linz, which is pretty much 1/2 way in between Vienna and Salzburg. It's a chilling, educational, important and memorable experience.