I know it’s not proofgrade, but I see that BF Plastics is having a sell on Romark-brand acrylic sheets. (http://www.bfplasticsinc.com/news/Garage-Sale.asp?page=1)
It’s all 1/16", some laser, some laser-iffy. Price at $10 for a 2’x4’ sheet is pretty good. Think you have to have an account to order, but it wasn’t too hard to set up.
There’s also a Romark page that talks about laser capabilities of their different lines. (http://www.rowmark.com/MARK/laser_guide/pages/prod.asp)