To make a low cost batman mask (also called a cowl), weapons, costume, armor, or anything Batman related, I would use a technique called pepakura. It’s kind of like origami but on a very large scale. There are tons of pepakura sources and helps if you are interested.
My Pepakura 101 Post explains pepakura a little more and shows where you can find tutorials that will really walk you through the pepakura process, tips to succeed, and other info you may need.
The Hero Tutorials mostly shows how to use pepakura to build an iron man costume but the same principles apply to a Batman build, so I highly recommend watching his stuff to get an idea of what materials you’ll need and how the process works.
Batman Pepakura Files
TheRPF.com – Batman Cowl, Chest piece, and Shoulder
Another TheRPF.com Location – Full TDK Suit: Material list, estimated cost, and pepakura files for The Dark Knight in paper and foam
Other Batman Sources
This site is just cool. It has a Batman Desert Storm Costume File but it’s for sale…only $250! That’s really not a bad price, it’s just more than many are willing to pay when there are some free files online.
Brotherhood of the Bat Website is a forum just for batman fans. It has lots of pictures and help for those building batman costumes and props.