Jef,
 Yes Corning 703, 705 is very exspensive. I use 60 lb rockwool made by Roxol. I get it thru ats acoustics and make my own panels. Easy to do.  http://www.atsacoustics.com/  go to the DIY page to pick what you want. I use the ridged rockwool since I use it as bass traps and they need to be able to stand up and not flop over when standing on end. They also offer already made panels. So do other sites.
Do yourself a favor and down load Room EQ wizard from Home theater shack. Make some measurements to see what you need and how it is changing your rooms acoustics. REW is free software.
Also don't forget you need enough bass trapping and absorption or diffusion on the first reflection points to make a difference. One bass trap will do nothing. I really think it hard to over do bass trapping. Do as many corners you can. Don't forget any point when the walls, floor and ceiling meet are corners.