Drawflower sells reusable lunch bags for $19.50. They are cute, but they got me thinking about making your own for dirt cheap. Here’s a tutorial of how to make the above bag from Tumbling Blocks.
The key would be to go to the local fabric store and see what you could find in the clearance section. Vinyl/oil cloth/etc. is usually on sale, and you wouldn’t need too much of it. Add that, some thread, some velcro, and a little bit of labor and you’ve got a lunch bag that can be used over and over again–plus it might make you want to bring your lunch to work.