there is one very significant difference: The cheap one is highly flamable. It's your life/gear.

Here's one place As far as being flame retardant, from their site: ALL acoustical products (charcoal & colors) meet a Class B, California 117 flame retardant specifications. Generally, this means if you were to briefly expose our foam to a flame, the foam will self-extinguish. (NOTE: We do not encourage anyone to try flame tests themselves. We don't want anyone getting hurt. Also, please check building codes before installing our acoustic foam, because different building codes apply to different locations.

I purchased all my acoustic foam from the Foambymail folks. I highly recommend, especially after paying waaaay too much for Auralex in the past 6 rooms I built. My only complaint is that the cuts are not as evenly matched as the Auralex product but I was able to find enough matching pieces to have it look nice while still spending fewer dollars. Do a search on eBay for "acoustic foam" and you'll find the foambymail folks selling slightly cheaper than their web site prices. I know you don't know me from Adam but it's the same stuff Auralex is selling, just a lot cheaper. Peace.

Thanks for all the advice! It's good to know that you don't have to pay for the brand name on this stuff... I'm probably going to be building an iso box to shove my amp in, and if/when that happens, I'll dig this post up and add my experience to the list.