looking for a yard or two of something cheap

Any quality fabric shop will have a $1/yd bin. At the very least you can prototype some designs with it. My fabric shop actually carries parachute material in their dollar bin. I was lucky enough to find plenty of silnylon and uncoated nylon to practice on since it has a considerably different feel (slippery) than most other cheap fabrics.

I also hear that walmart has a similar dollar bin for fabric, however I've never seen any walmart in my area that sells any fabric at all, but many other areas apparently do.

Good luck!

Try some Tyvek,
You might even be surprised at the durability!

You can pick up XPAC locally, Loveland, for half the price of Dyneema.
http://www.rockywoods.com/Fabrics-Kits/All-Ripstop-Nylon-Fabrics/XPAC-Ripstop-Nylon-Fabric
It's slightly heavier but just as durable for your application and easier to work with. You should be able to make at least two frame packs from a yard of fabric and have extra left over.

I have a yard of 330d Cordura in the gear swap!