We had no budget. My panel cost is 1/6th that of a commercial panel. Found a carpenter in the congregation. Built the first test panels with him, donated the foam. The improvement was enough to get the music lovers rolling. Funding was found.
Second batch, Corning was too expensive at the time, we switched to the Rock Wool. Which is available locally in much of the world. Half the cost per panel, but takes more time to assemble.
The simplest Dayton reference mic with calibration file costs perhaps one half of your DSP board. IMM-6 costs 27$. If your willing to make some assumptions, any electret mic element will work.
IMM-6, plugged into an Apple product, gives you really accurate levels, as Apple calibrates their tablets/IPods on the production line.
The one third octave equalizer was 99$. Notching out the feedback causing 406 Hz tone from the female vocalists, priceless.
Break one corner, even if you use only one panel straddling the corner, note the difference. Usually you only need to do one or two corners.
Rock wool, made from slag and minerals similar to glass, is dirt cheap and does not burn. Standard pink attic insulation works well, but only has 1/3rd the acoustic density, so you need a lot of it.
Guilford was used due to its fire rating, Burlap works just fine. However in a brick building with wood arched framing, and no sprinklers, fire resistance was on my mind.
I too, live on a budget. There are plenty of web sites on home made acoustic treatment. That is where I started.
Good luck... let us know how your system works, because I assure you, there is a demand for such things.
Steve