Foam isn't really cheaper..
Auralex 2" Studiofoam Wedgies (1'x1' 24-pack, Charcoal) | Sweetwater.com
^ $110 for 24 sq feet
We sell 48 square feet of rigid fiberglass for cheaper than that. So twice as much fiberglass, and still paying less. (Not trying to sell our product btw - check other competitors too - still cheaper than foam)
I know not all foam companies charge as much as Auralex but really the price difference isn't huge - and fiberglass is still usually cheaper. Also factor in that the studio foam is only 2" thick at the peak, so really averaged out its only 1" to 1.25" thick.
Contrary to popular belief - not many pro studios use foam in vocal booths - and I'm not quite sure where this idea started. Professional designers hardly design rooms with foam. It kills reverberation yes, but leaves muddy low end from standing waves intact, so you still won't end up with a treated room. Getting down to lower frequencies with fiberglass or mineral wool would be a much better option. Also, a professional vocal booth is typically bigger than most people's bedrooms, let alone closets!
BTW - the flat nature of fiberglass plus the bonding agents does make it seem like it is reflective, but it really hardly is reflective at all. For a live room, diffusion or covering absorption with slats would keep the room live. Fiberglass itself doesn't tend to help keep a room more live.