Yeah, I checked prices for ripstop nylon when I posted. Cordura doesn't seem to be a whole lot more expensive than I said.

http://www.rockywoods.com/Fabrics-Kits/1000D-Cordura-Nylon

It's also heavy. A better choice for this person's application would be Tyvek. (e.g., https://backpackinglight.com/forums/topic/18758/ ).

Maybe the licenses for the patterns I was buying it with pushed my prices up.

It's also incredibly heavy. There's a reason why no ultralight backpacks use the 600-1000D Cordura that more robust bags are made of.

You can find it very cheaply in some cases. I once called a ripstop manufacturer and got 75 yards of slightly miscolored fabric for free.

Tyvek would probably make more sense for a project like this. $30 will get you 10 yards on Amazon

Tyvek is loud. I used it to sew furniture covers for a cabin that is left unoccupied in winter. Shaking out the bed covers to put them on is deafening. Walking with a Tyvek pack might drive you mad.

Those IKEA bags are pretty freakin loud too. Both are materials I wouldn't want near me.

I don't think tyvek would be strong enough for a backpack. It is used a lot in backpacking for ground sheets and even shelters.

Tyvek also breaks down in UV light.