Just to clarify for those who don't want to search, mineral wool (or rockwool) is definitely usable for acoustic treatment (very similar acoustical properties to OC 703). The main issue is that it may be a little bit more difficult to work with, but it is quite a bit cheaper.
I was wondering the same thing, would anybody happen to know if roxul safe sound would work, i heard it does the same exact thing as 703, but it's cheaper