I agree the economics simply is not there anymore for DIY solar heater construction, with commercial panels being as cheap as they are, particularly if bought online. I just bought 11 good brand name 4x12 panels complete with mounting hardware (all the roof top parts, I already had the ground level plumbing and controller) for right at $270 each delivered. Going with cheap panels that would still likely be better than anything you could make it could probably be done for under $200 each. Like Jason said it is all about coverage area with solar, the panels I bought each have 104 water channels inside the semi-rigid flat panel (the collectors look a bit like a corogated plastic yard sign, but beefier), that means to they are like having over 1200 feet of small diameter black tubing neatly laid out side by side on each panel all set up in parallel so you don't get excessive flow resistance, you just can't approach that in a DIY construction for the money.
DIY installation of commercial panels on the other hand can be very economical