If you have a 55g tank, you can take it apart and use the glass to make a 100 for the cost of 1 sheet of plywood plus waterproofing and screws.
If you look on freecycle and craigslist you can find 55's for free/nearly free. (I've gotten 2 55's for free on freecycle, and a 150 for $100 on craigslist, but it took about 6 months of looking/waiting)
I think someone here used a 55 to make an 8 x 24 x 24 with 2 front windows.
Besides the "I did it myself" being able to make custom sizes/shapes is the real benefit of DIY to me.