I checked prices on the biggest austrian motobike site (1000ps.at)
they also list rental bikes from most companys in austria
for example, they list a BMW F800GS for 120euros/day, 700km free. and if you rent it for 4-6 days, you can get it for 110euros/day.
altough i think its the same company that you inquired.
edit: you might wanna check the weather forecast. doesn´t look good this year, its way colder then normal for this time of the year. and it should stay like this for the next two weeks. maybe you should look for a bike with good wind protection and heated grips/gear...
2nd edit: here´s a link to a weatherforecast for the town of Mariazell, the most popular spot for riders around here: 16 day weatherforecast for mariazell