Call your local glass supply place (automotive glass, window glass, etc..), and they may be able to help you out. They will cut it to size for less money. I just got 5 pieces of 1/4" glass cut to cover ~ 4ftx6ft for $72. Home depot's prices have a "convenience surcharge" applied for those that don't know where else to go.
same should apply to plexi as they will do alot of that too.