Great deal by my book. New sheets from vendors run from $350 to $550, cast, 1/2", 4x8.
The warped sheet will give you problems at assembly, as stated above, which can be overcome if you are experienced and resourceful and the tank doesn't need to look perfect, just be functional.
I have had 16 sheets of warped acrylic 3/4", 4x4, under a ton of bricks for the last several months to see if they would straighten. Haven't checked yet.
1/2" was used for my friend's 480 gal 8x4x2, professionally made. It will surely hold water but will bow a LOT, so don't be surprised. It may lead to eventual crazing sooner or later.