Several months ago I bought a glass 20gal high tank to use as a sump tank, think it cost about $45. That was for just the tank, no top or light. If you can locate a local fish club or aquarium society and go to one of the meetings you will often see tanks auctioned off for a fraction of their cost new.
I've read a number of posts from those who have made small tanks under 10gal out of window glass sold at Home Depot. I don't think this would be a good idea for larger tanks, because these designs used thin glass and were only held together by the sealant in the corners.
I've read a number of posts from those who have made larger tanks out of wood with a window in one side which eventually worked, but they had a difficult time sealing all the leaks.
You might be better of getting a manufactured tank and using DIY for the accessories such as lid, lights and filter. If you go this route, the best advice I can give is that if you use plexiglass for the lid use something thicker than 1/8" or add a center brace because the lid I made for my 20gal sump bows considerably under its own weight.