This video covers the whole process of building broadband DIY acoustic panels for room treatment.
Each panel breaks down to $25 but since rockwool safe and sound 16“ comes in packs, I recommend building 8 which comes out to $200.
You can find the cost breakdown, measurements and links to the materials I used here.
00:33 Measurements
1:09 Cost Breakkdown
2:37 Tools Used
3:05 Wood Cuts (if you used particle board)
3:18 Frame Assembly
4:38 Fabric Wrap
7:07 Mounting