Sounds like you guys are the perfect candidate for the
DB Bayern Ticket. €25 for the first passenger, €7 for each additional person (up to a total of 5 people) will let you travel virtually anywhere in Bavaria for the day, including to "border" stations, of which Salzburg is one. You can only use local public transportation and regional trains (no IC/ICE/RJ), but it's a great value for a group of 3-5.