Lowes has 4' x 8' sheets of 1/2" PVC board for $80.00... But, yes, you can find perfectly good 4' x 2' enclosures on the market for as low as $160.00. I see them all the time at the reptile expos for $225.00. For the cheaper enclosures like that you can definitely buy them for almost as cheap as it would cost to build them. The little extra it would cost to buy the cheaper ones would be well worth not having to mess around building your own for sure!
Check this site out:
http://boamaster.com/
That site has their economy 4' x 2' x 12" PVC enclosures for $175.00 SHIPPED. That's pretty hard to beat! It would cost somewhere in that same ball park to build one. Have any of you guys had any experience with that company and their enclosures?