the short answer is, no. the price of building your own vs buying one are very similar you might save 5 - 10% but you run the risk of assembly problems, catastrophic failure, etc and by and large glass prices are based on location, the price a guy from new york will pay, is different from what someone in washington will pay. about 10 - 15 yrs ago building your own was a great way to save money, but now so many tanks are built each day by companies, the price you pay in store has fallen greatly. other option is hunt around your local cities for an acrylic supplier, they might cut you a good deal on surplus stock, mis cut stock, etc and the savings might make acrylic cheaper than glass.
if you were going to build a plywood tank you might save about 15 - 20% and is probably the 'cheapest' route to go, since you are only buying 1 piece of glass. the only thing that would bump up the cost is marine grade sealer which from my research price varies on location, and availability (not to many people are going have use for boat sealer in utah). however for a plywood tank all cuts have to be perfect (more so than an all glass tank) and construction time is considerably increased.
the only reason to build your own tank vs buying a prefab unit, is if you want some crazy dimension, a weird shape, want a cheaper acrylic tank [building your own acrylic tank is typically more expensive than the glass alternitive, but by building your own, you can beat the 'sticker' price of an acrylic tank] or if you have the materials and the know-how laying around your house.
however to some people, saving 5% is more than enough reason to build your own, and not to mention the thrill and satisfaction of building your own tank. if you are dedicated to building one, then you need to do some research, i would call every glass shop in your area, and ask them the price for the dimensions you want, and maybe a few hardware stores too (not home depot, a true hardware store - some will have a small glass area in store, with limited selections, in the cleveland area, i know of 2 hardware stores like this) and then take the numbers they give you and figure out what the total cost would be, then make your decision, most of the time though people opt for buying pre-fab. and do not forgot any tank over 55 gallons will need a tempered glass bottom or a thicker piece of glass.