We stayed at the Graben Hotel in Vienna about a year ago. We paid right around 130 Euros a night. It was in a great location, and was a nice, decent hotel - nothing extremely fancy but nothing shabby either.

As far as your dates, remember Oktoberfest will be going on from 9/17 until 10/3. So, you may either want to avoid those dates completely, or go right in the middle of them, depending on if you want the Oktoberfest experience (which I would highly recommend). The weather in September can be great - just like fall here. If it were me, I would choose September over August for the weather and Oktoberfest.

Just my 2 cents.