I don't know their NFA transfer fee, but I was planning on using Merrimack Firearms because they have a SilencerShop Kiosk, so the process is simplified and there's no transfer fee if ordering from SilencerShop.
I'm sure others with more experience will chime in to assist, but it couldn't hurt to call a few shops and ask for their fee schedule .
NH Guns and Ammo in Londonderry is awesome for transfers. I'm not sure about their NFA transfer price but I know it's $15 each for non-NFA with a max of $30 if you are transferring multiple guns. I was in there this weekend and went over some suppressor options with them and they were very patient with this NFA noob. The guys there are very welcoming and friendly, and don't look down on you for buying elsewhere and transferring through them like some shops do. They aren't the closest shop to me, but they are definitely my go-to from now on.