Salt mines...on your own by city bus? Possible?

I'll answer this question now myself. Yes. You can take the city bus to Berchtesgaden...then get off 2 stops sooner at Salzbergwerk. So fun!!! (And much cheaper!)

take the city bus to Berchtesgaden

That "city bus" is the Walzmann Express, RVO 840, which is actually a German bus owned by Deutsche Bahn. RVO buses run all over upper Bavaria, with just a few routes outside Germany. This is one of them. It goes from the Salzburg Hbf, with some stops in town (Marienplatz, Mozart's Bridge), to Berchtesgaden Hbf.

Here is the schedule.

There is another Salt Mine, possibly bigger, in Hallein. You can get to Hallein from Salzburg by train, about 25 min. There is a bus, # 41, from there to the Bad Dürnberg Salzbergwerk (11 min).

A one way ticket from Salzburg to Berchtesgaden will probably sell for about 5€ each way, but you can buy an RVO Tagesticket from the driver for 9,80}. It will cover the round trip from Salzburg to Berchtesgaden plus use of most all of the other buses in Berchtesgaden.

The tourist office at the train station sells a combo ticket to the Hallein salt mine - train, bus, entry and return

To be honest, I've never been that interested in seeing salt mines. Added to that, there are so many more interesting things to see in Berchtesgaden; I've been there four times and never been tempted to see the salt mines.

Take a boat ride on the Königsee, be sure to go all the way to the end, to Salet. The Obersee, on the other side is beautiful.

Go up to the Kehlsteinhaus (Eagles Nest). I found it eerie to know I was in the same room that Hitler had been in.

The town of Berchtesgaden, itself, is worth walking around in.

Take the bus out towards Ramsau and hike in the Wimbachklamm.