Oh, I hadn't, but probably 4-6 bucks each. It all really varies. Canvas can be made of all sorts of materials I think, and it depends how desperate a factory you find. Double, maybe triple the price of the nylon at any rate. Most of the numbers I threw out there were my most medium guesses. Stuff fluctuates a lot depending on where the factory is and what scrap materials are available to be sourced. There's a ton of leftovers for making stuff like this that doesn't have to be FDA approved or sterile or whatnot for science or food purposes that factories utilize.
Edit to add:
My estimate was way off. Once you start adding straps and crap to it, like theirs has, the price of canvas goes crazy. Take these prices as about 1/3 lower after negotiations, but here are a few examples of what Canvas bags cost:
https://www.alibaba.com/product-deta...4d9c4839B81wLi
https://www.alibaba.com/product-deta...4839B81wLi&s=p
https://www.alibaba.com/product-deta...4d9c4839B81wLi
Compare that to the much lower "basic" canvas bag, which is probably what they based their initial cost estimates on:
https://www.alibaba.com/product-deta...5be3fQB0r2&s=p
Theirs is probably somewhere in between those in complexity. Probably around $10-13 is where they'd land after negotiations.