I would be very surprised if you can buy new glass for lessthan the cost of a ready built tank. If you can get salvage glass at a reduced cost, or free than it might make sense. I have built a number of tanks, although they are all smaller than 125, but the glass was free. I was talking with a guy a couple of weeks ago, and he was offered 1/2" x 2' x 4' pieces of glass for $15.00 each (had to take all the pieces and stopped counting at 15). To cut the end pieces more easily, 6 pieces would be needed, but $90 for a tank of around 120 gals is cheap, although I would prefer a longer tank of that size. Still, It would be wortwhile.
Join a fish club. Don't have one in your area? Start one.