Just an FYI if you are shopping or wanting a current state of the market. Got some quotes on Acrylic from Allied and Regal Plastics.
Allied guy said their prices were higher so they could always keep some in stock. Regal must have better supplier to be able to maintain their prices which match up more closely with the prices I see online for similar products.
1/4" Extruded Clear - Full Sheet: $225
3/8" Cast Clear - Full Sheet: $350
1/4" Unknown Clear - Full Sheet: $160
3/8" Unknown Clear - Full Sheet: $315