unless you have an extremely good source to get the glass cheap then the savings in building it yourself is negligible but there certainly would be some savings. The other issue would be how nice the finish would be in the end in regards to the joints, seals, edges etc. The quality of the finish would almost certainly be less without the necessary equipment that a manufacturer would have.
that being said, I had a customer recently who built his own 180 gallon tank at a cost of $475.00 he said. Retail cost on the same tank is around $700.00 and his tank was well done and most people would never have guessed it was "home made". He did say however that he would never do it again as the time it took wasn't worth the savings.
If you have the time, the equipment, a good source for glass and are planning on building a larger more detailed tank with holes, overflows etc. then the savings could be worth it, just cross your fingers it doesn't leak or even worse, a panel doesn't let go on you! Lol