The thickest acrylic panel ever manufactured is about 2.5 feet thick. It was used at the Dubai Aquarium that opened up recently. The next thickest acrylic panels are about 2 ft thick. One at the Georgia Aquarium and the other at Okinawa Churaumi Aquarium.
If your acrylic supplier claims that they can get you 3 foot thick panels for $14,500, then I highly question how they got that estimate.
If you want a rough estimate of just acrylic resin cost alone before cell casting manufacturing process, you can figure that out with these resin prices-
http://www.ides.com/resinprice/resinpricingreport.asp
The resin alone for each panel would cost $38,654 If you get 5 panels = $193.273 and this does not include freight or cell casting manufacturing costs.
I doubt that anyone can manufacture 3ft thick acrylic anyway.
Even if you get a good price on the acrylic resin, like around $1.20 per pound, you still have to realize that you'll need 40 to 80 thousand pounds of it. Best of luck.