Have you seen the Wrap n Mat?  It is a reuseable Sandwich bag that runs about $8 each.  The average sandwich baggie costs $.05 each.  You would need to use your Wrap N Mat 160 times just to break even.  On the plus side, it is already made, and the lining is guaranteed lead free, PBA free, phthalate-freephthalate-free, and food safe.

A twist on the wrap in mat, is a rip-stop nylon lined bag with velcro closure found here:  http://www.etsy.com/shop.php?user_id=6941342.  The cost is simlar to a Wrap in Mat, but is plastic free and made by a Work at Home Mom!  The fabrics are definintely cuter too.

If you can sew, your opportunity to save money is expanded!  There are a couple of tutorials out there to make your own reusable lunch ware.

http://frenchsleepdeprivationstudy.blogspot.com/2009/06/snack-bag-tutorial.html

This tutorial tells you every detail on how to recycle mylar snack bags (like potato chips come in) into your own sandwich bags.  These use elastic and buttons to close which makes washing them easier than with velcro.  They are super cute too!

This one recommends PUL for the lining, but you could substitute nylon fabric if that makes you feel better.

http://wewilsons.blogspot.com/2009/02/reusable-snack-bag-tutorial.html

Or try this one.  It's a unique shape that looks easy to make and use.

http://www.craftstylish.com/item/43227/how-to-make-a-reusable-sandwich-wrap

This last one I love, because it uses a freezer weight zip-loc baggie and turns it into a washable sandwich wrap.  It's easier to wash than a traditional baggie, is already food grade, and will last a loooong time.

http://www.towards-sustainability.com/2008/03/homemade-sandwich-wrap.html