[Effendi]

Of course roaming in Austria with a German card involves high roaming fees. I think all German sim cards are VERY expensive there. So, buy the card of the country you are going to spend more time in, or buy 2 sim-cards, one German, one Austrian. For the Austrian I think Yesss could be the best choice, it's a virtuanl operator on GSM1800, good coverage, and you can buy it at many gas-stations or supermarkets.
About Germany I think e-plus has good 1800 coverage, surely it covers Garmisch-Partenkirchen, maybe they don't have the best coverage up on the mountains, but usually there are no problems. $40 for a T-Mob card are too many, you can get a new one in Germany for ? 20.