When it comes to either rockwool or Owens Corning 703, don't buy them on Amazon. They'll ding you with shipping costs. Even on Amazon.
So to avoid shipping costs, go to your local hardware store. ie. Lowes or Home Depot
In most cases, hardware stores will have one or the other. But rarely both. So they have either rockwool or thermafiber or Owens Corning 703.
If they don't have 703, go for Rockwool.
So the total cost should be inexpensive. You can get Thermafiber for about $43 or Safe N Sound for about $50 for a HUGE bundle.
If you can't bother with the labor, there is another way which provides amazing sound treatment and requires no glue, no measurement, no saw, no drill, no spraying on end for 10 minutes, no wood, no staples, no sandpaper. None of this tedious work which takes hours on end of drilling, sawing, measuring, etc.
Just two things. A body pillow cover with a zipper. And Rockwool. Stuff it in. Zip it in. Wipe it down with a wet rag.
Takes minutes. Rather than days.
Looks very janky. But provides spectacular sound treatment. Just be sure to have gloves and a mask. Double up. (Use two 3 inch blocks per body pillow cover). The first one goes in very easy. The second, you need like a cardboard placement or something in between to make it slip in.
Instructions:
Step 1.
Take out one of the Thermafiber insulations
Step 2
Put it inside the body pillow cover. You can fit 2 in each body pillow.
Step 3
Zip it up
Step 4
Clean excess crap.
Hang as needed. Performance is seemingly identical to acoustic panels (built) but takes only 3 minutes to do.