My 50 cents, all IMO...
I think deciding whether you use a professional frame shop, assemble pre-cut components yourself, cut and assemble all of the components yourself, or some combination of the last two, depends on what you enjoy (or at least don't hate), what you need to accomplish, and economics. Due to the equipment expense and learning curve it makes no sense to cut and assemble mats, mounts, glass, and frame moldings yourself unless you really enjoy it or you need to produce quite a few framed prints on a continuing basis. The latter means you are probably selling the prints. That is a dubious endeavor but it can mean your equipment costs become insignificant as they are spread over many prints.
Selling prints at a full service gallery often means paying a 50% commission. Some take less but pricing needs to account for this worst case. Changing the selling price based on the venue is a horrible business practice that will eventually catch up with you. Consignment agreements at many galleries require consigned items to be sold at the same price everywhere they are offered. Selling only at arts fairs and festivals may seem cheaper, but what you spend in time, jury fees, booth fees, amortized equipment (tent, display panels, print racks) and inventory quickly makes up for the lack of a commission. Roughly speaking I'd say the outdoor venues are equally expensive, but the sales volume is higher.
Considering the 50% commission or its equivalent, every dollar saved doing the work yourself means $2 added to your bottom line if the print sells. For example, suppose the total cost for a professionally framed A2 size range print is $150, pre-cut materials for the same print are are $100, and doing the whole job yourself costs $70. With a 50% commission the break even selling price is $300, $200, and $140 respectively. If in your area such prints will sell at $400, you make $100, $200, and $260 respectively for your time and talent on each one. Over years and decades even the $60 difference becomes very significant. FWIW, I made up these numbers but I think they're reasonably close, and the principal holds regardless.
You can reduce the cost of pre-cut mats and frames by standardizing on particular sizes and buying in quantity. The down side is cropping at least some images to aspect ratios that are not optimal. I find the 2:3 aspect ratio of the camera often works, but at least 25% or 30% of the time it is not ideal. I end up with other "standard" aspect ratios like 4:5 and 1:1, but not infrequently something really odd and one-off works best for the image. Cutting all of the materials yourself means the cost does not increase when a non-standard size comes along, and the mat is not ill-proportioned to make the odd-sized print fit into a standard frame. The ability to produce an odd-sized mat by the time the ink on the print has dried also comes in handy sometimes. You can't change the price at which your prints will sell at a reasonable frequency, the commission charged by galleries, or the cost of doing arts festivals. One of very few things over which you have some control is the cost of producing your work.
As I said starting off, the whole endeavor is dubious. Most of us would have to participate in an arts festival nearly every weekend of the year, live like a gypsy because of it, and have work in numerous galleries to make a great living selling prints. In that case there would be little time to take photos and make the prints, so it's nearly impossible. I make prints because I love doing it, but my house can't possibly hold everything I want to make, and there are always more. That necessitates selling them or piling them up unseen until I have enough for a good bonfire. I sell them because it perpetuates and pays for what I love doing (making the prints) and what I don't hate doing (cutting mats, glass, and selling), with some pocket change left over for other photography related stuff. I wouldn't do any of this if I didn't love it, but I also wouldn't do it if it didn't at least pay for itself. For me it takes both and doing 100% of the work myself helps achieve the latter. YMMV
