Although it is not very difficult to build a tank, it is not really cost-effective unless you happen have the materials on hand already. One problem is that it is very difficult to find information on the correct thickness of glass or plexi to use. Generally, if your tank is taller than 16 inches, you need thicker glass. I read once that you can build tanks that are less than 16 inches deep with 1/4 inch glass with no problems, but with a really long tank, that won't work without bracing. I have built sumps from plexiglass, but it is tough to get a really smooth edge without an expensive plastic cutting saw blade. Gluing is pretty easy. You'll find that thick glass is hard to find, and very expensive. It's usually cheaper to buy a tank, unless you're looking for something really unusual in shape or size. At one of the big chain pet stores here you can buy a 125 gallon tank for less than $300. I think that's an All-glass tank. They're banking on you also buying the stand, canopy, filter, and everything else from them.
Sonny