I agree that the cost will likely be a little more than 100 euros from a group of 5. If your parents are still good for walking a bit, I would highly recommend the Hotel Erzherzog Rainer in Vienna. It's about a 10-12 minute walk south from the Opera. It was much less expensive than any of the comparable quality hotels right inside the Ringstrasse. The rooms were very nice, free wifi, and the free breakfast was honestly one of the best, if not the best, hotel breakfasts I've ever had. It even had complementary champagne! For the two of us, it was about 80 euros per night.
One note though is that it does not have air conditioning. We went in May and had no problems, we just got a fan and cracked the windows. But if you were going in July or August, I could see this being more of an issue.