I have been looking into this alot lately and it depends on what kind of tank you want. I have been looking at making a tank, either plywood or all glass, and some where around 120G-240G. and so far it is turning out (for me at least) that it is the size of the tank you want that dictates which is the cheapest route. I have been calling all over at my local glass shops pricing out glass for an all glass aquarium, and the glass is not cheap! so it is looking to me like you only really save money making your own tank if you want a REALLY big tank, or a funky shaped custom tank. But if you are looking for one of the common big rectangular sizes....ie 120, 180, 210, even 240's. you can find one for around the same cost of building one, maybe even cheaper, not to mention that you don't have to build crap and the tank (should) be high quality. so if your looking for a 120-200 I'd say buy one. but then again that is the cost breakdowns I have come up with so far, and other people can prolly get better deals on glass and such.
the high cost of the glass got me thinking...."why in the hell do I need to spend $300 on a back peice of glass that I am gonna cover/paint anyway??" so that got me looking into the plywood thing. but the height of your tank is vital when choosing glass thinkness. I really wanted it to be 36" tall so I needed at least 1/2" glass (according to my online research) and one piece of 1/2" glass at 96"x36" is very expensive. when considering the price of a 240 long from glasscages....I got off the DIY route real fast. but thats just me.