Check out GARF, I was looking around there today and found a prop tank made of 4mm coroplast. I didn't look at it closely, but it would be super cheap. Last time I bought coro it was about $15 for a 4x8 sheet.
http://www.garf.org/_2008b/show/index.htm
African Fever on Alberta Aquatica has built a couple large plywood tanks. In one thread he gives a breakdown of materials and costs of what he used. Both threads are a good read for information. He's a good guy and would answer any questions you may have about plywood tanks.
http://albertaaquatica.com/index.php...l=plywood+tank
http://albertaaquatica.com/index.php...l=plywood+tank
HTH