I can't tell you what the average cost is; but I can't imagine any lawyer would do it for $100. My normal billing rate is $200/hr. In order to do an NFA Trust, I would need to meet with you, determine whether a basic NFA trust was appropriate to your needs, discuss what type of assets you wish to place in the trust, draw up a contract/get you to sign a disclaimer indicating you understand the limited nature of this trust. Once we had that done, I'd enter you into the system so that I had a computer record I could reference should I need to update your trust for some reason and I'd modify one of the existing forms I created to suit your particular needs. Even for a very experienced lawyer (who don't often bill at $200/hr by the way) and a very basic form, I do not know a way to do that ethically/responsibly in 30 minutes.
Even though I am licensed to practice in Texas and have been approached a couple of times to do NFA Trusts, I don't do them; because it doesn't make economic sense for me. I've already got more work than I can physically do at my current rate and gun owners don't want to pay my current rate - and for that matter, it is more appealing to deal with three or four corporate clients with their own legal departments (often staffed by people I went to school with) than to deal with several hundred small clients who aren't familiar with the legal system and need a lot more explanation.
I once toyed with the idea of trying to offer a very basic NFA Trust for $300; but in the end I felt like the potential downfalls weren't worth the cash - and even at that price, most of the people I talked to were not at all interested.
As for whether your friend got a good deal or not... impossible to say without looking at all the paperwork. Trusts can be very simple, no-frills, 1 or 2 page instruments made for a very narrow, specific purpose or they can be binders of documents designed to handle tax consequences, incapacity, probate, etc.
I could go on at length about the business aspects of doing NFA Trusts. It really doesn't make a lot of economic sense for most lawyers unless you really get out there and hustle it or you use it to draw in big clients (i.e. machinegun owners) who tend to have the kind of assets that generate future work for the firm.