<<Using the canvas drop cloths for a tent, that's a good idea. I'm guessing that it is cheaper that way than buying yardage?>>
Definitely. We got two 12x15 drop clothes for like $76 (although I was dismayed to find that the measurement is not correct; they cut the fabric out that size, then sew it up and you lose inches in the seams--I asked my husband if that wasn't like saying that a pair of pants was a size 36, except that they only fit someone with a 34" waist? He said take it up with the tarp industry).
Anyways, I cut them in half and so I got four nearly 12' wide panels, nearly 7.5' long--our walls being 7 feet long or a bit less. I also had to add in a section of 4 foot-wide canvas that I already had (and bought at a bargain price because it was a remnant) to give us enough coverage (50' total) and a bit of overlap around the corners and things like that.
If you get canvas that's 60" wide, you'd need 10 panels of it (think of the sewing labor involved in that--all I had to do was hem one raw edge on each panel and attach clips), each 7 feet and a couple of inches long--so say 2.5 yards. That's 25 yards of fabric. At $6 a yard, that's $150. And that's the absolute cheapest we saw any canvas.
So, considering our remnant cost us like $6, I think, we only spent $82 total on canvas--compare to $150. Plus, as I mentioned, it saved a LOT of labor to buy such large sections already put together and hemmed.
We have a friend who has made her own canvas tent before (and we're talking large, fancy tent here; not a piece of canvas thrown over a tree limb!) and I thought she was crazy for doing it, but, actually, I found that the canvas really wasn't that hard to sew after all. The worst of it was trying to get it folded in half so I could cut it; it was wet and muddy outside--had been for weeks--and the drop cloth was bigger than our kitchen, even when folded in half. So I had some difficulties cutting out something that large, but it wouldn't have been so bad if I could have taken it out to the yard to do it.