what is the roundtrip price to mondsee from Salzburg? and where do I get bus 140?
Go to the OBB website and you can book a ticket. It should show a price and you can cancel after checking. The bus 140 should depart Salzburg Hbf:
http://www.oebb.at/en/index.jsp#
Five years ago, I took the bus from Salzburg Hbf to Mondsee. Back then, the one-way fare was 5,60€. I don't think there is a discount for a round trip, so figure about 12€, maybe a little more today. There is a bus plaza in front of the Salzburg Hbf. The bus slip was clearly marked.
Update: According to the fare page on the Salzburger VerkehrsVerbund website, www.svv-info.at, the one-way fare from Salzburg to Mondsee is now 6,30€, per person.
Bus tickets are purchased from the driver; you cannot purchase these online.
Nevertheless, you can find out online what the fare from the bus driver will be.
Just carry plenty of cash and throw it in the driver's direction ;-)