Ownes Corning 705 2" thick. Cover it with your choice of fabric. You can find it on Amazon 4 pieces for about $100.
Yup. I used something similar -- Rockwool Safe'N'Sound bought at Lowe's. $60 for a package of twelve 16"x48"x3" batts. Built frames out of 1x4's, covered them with burlap fabric. Made 4 panels for less than $100 with material leftover to make more down the road.
I did buy some corner bass traps from GIK, but based on the absurd delivery time and quality issues I had once I finally got the product, I will likely build additional traps myself.
The prices most of these acoustic panel manufacturers charge, compared to the actual cost of the raw materials, is kind of obscene. If you're even remotely handy, I would encourage you to DIY.