Well, I guess technically I'm not just a "frame" store, the mounting is part of our printing services, not framing, so perhaps a little lower priced. We charge $15 to mount a 16x20 to thick foam core, $18 for gator. We do get some prints brought to us for mounting and we're happy to do so as long as they sign a release.
As far as brush on adhesive, perhaps I should rephrase my statement...
I wouldn’t use a spray or brush on adhesive for any fine art paper, only heat or cold adhesive mount. However, if I HAD to use one of those two, I'd probably opt for the spray on.
The peel and stick options are actually pretty good for prints 16x20 and smaller, obviously the problem is having to buy a packet.
(seems like a lot of effort to save a few bucks, $16 plus mount board might be worth saving the hassle and faster ...)