That's a fair question

It is definitely possible if you aren't picky about what it looks like. If you're near a Wal Mart that still has a fabric dept, try there. They always have a table of $1-2/yd fabrics, and often you can find heavier canvas, twill, or home dec facrics there. When I made my bag I found some weird, shiny, black denim for $1/yd. It was stiff and scratchy, but who cares if you're not wearing it? Since it was 60" wide, I could cut it out with the long part of the bag going across the width of the fabric, so I only needed 1yd to go "around" the stroller (hope that makes sense, I'm terrible at explaining it in words) If you don't have a WM w/ a fabric dept, try the remnant section of a facric store, especially the home decorator remnants. They tend to be very sturdy fabrics, usually 60" wide or more, and larger peices - 3-4+yds.
So, I spent $1 on fabric. The other supplies I needed were some cord for the draw string (free, found something suitable in my junk drawer), one of those little cord lock thingies (around 25 cents), and 2 small d-rings for attaching a strap (around $1, I had to buy a 4-pack). I cheated a little with the strap. Instead of buying webbing for a permanent carry strap, I attached the d-rings and borrowed a detachable strap from a duffle bag I wasn't using for the trip. Quicker, easier, cheaper, and more comfortable to carry

So, my grand total for the project was less than $3.