There is much more to glass than thickness, but you knew that already.
Chopped matting has some key advantages. The primary advantage being the non-directionality of the fibers. This provides superior pinhole resistance, unlike a weave which can pinhole with the fibers get spread in an area, compromising the box seal.
There are two very key disadvantages to chopped matting. First is it cannot conform to compound corners. If I wanted a flat panel, I'd just use MDF anyways. We work around this by cutting strips or squares, and in extreme cases using loose fibers to make a slurry. Second, it DRINKS resin and fibers separate. This makes it super-easy to get a target panel thickness with a minimum of material, but super-hard to do it with the strongest glass-to-resin ratio. Weave or roving doesn't puff up, so you need more fiber to get it thick. But the strength is higher because all your strength of course comes from glass, not from resin. So matting is a false economy.
Beyond that, matting is a nightmare to finish. I remember many a discussion on CAF, ECA, and DIYMA about the best body fillers to use, the best techniques to pre-grind the box, obtain a smooth surface for filling, etc. What a nightmare! Doubly so when the matted fibers start poking up during the layup, creating a surface layer that looks like pinhead form hellraiser, and just as unpleasant to handle. That's fixable my making the surface layer a cloth mat, but it's going ot be bumpy because the under layers are a hodgepodge of matting squares. You'll probably find you don't even need to add resin to the surface cloth layer, it'll soak in from underneath because there is so much extra unused resin pooled into the matting.
------------
None of this was your question. Your real question, was is it cheaper? my guess is from a materials perspective, no. However, I used ZERO body filler on my box, and would have needed zero bodyfiller even if I had exposed fiberglass in compound corners on the top side. Good bodyfiller is expensive, so we're getting closer to parity.
Factor in labor hours, even though it's a labor of love, and it's not even close. I did the whole thing in ONE layup, and I was done after that. No building up layers, no sanding, grinding, filling the weave, fussing over this or that. It was: make the layup, press the base wood rings into the layup, ensure the wood is soaking up enough resin to adhere, and let cure. Pop out and done. No followup, no "milkshake", no fuss. Just filler-primer and paint.
I am blessed to have several TAP plastics close to me, so I don't pay shipping, only sales tax which in California is about 75-90% depending on your county and city.
-------
NOW, the elaborate material stackup was just for fun on my part. I could have easily bought a few yards of 8 ounce woven cloth and used only that material to build up my layers to get the desired thickness and off I go. I would have ended up with the same result. But it was more fun to experiment with novel materials, because this layup was frankly so easy. small compound corners, a relatively flat surface. It makes it easier to experiment with materials so I have the working familiarity with them should I want to do something more advanced in the future, tight compound corners, vertical surfaces, etc. So it was fun.
But you WILL NOT make a panel as strong as mine using only chopped matting, unless you make your panel thicker, heavier, and uglier. But yeah, it'll probably cheaper before you start using bodyfiller.