Just for giggles, I called regal yesterday (briefly tossed the idea around of making myself a tank...something I have been wanting to do for fun, but always punk out at the last minute). And I got the following prices on 4' x 8' sheets of acrylic (but couldn't tell you if this stuff is cell cast or not, didn't ask, and I didn't know any better to ask at the time):

1/4" thickness - $111
3/8" thickness - $202
1/2" thickness - $282

She said cutting to size should be another 10 - 15 bucks.