Building a tank is NOT cheaper than buying one, however there are advantages...
1. It's the exact size you want.
2. You have a project that will consume untold hours of time. good for those people who have fidgety hands like me.
3. You can build it slowly based on cash flow.
Lets see, costing out a simple 100 gal...
50$ for A/A Oak Plywood (personal choice but I like it so)
50-100$ for sealant (depending on what you buy)
35-120$ for glass (tank height, 3/8 is expensive and 1/2 is outrageous)
20-50$ for finishing materials such as stain, paint, silicon...
So, the minimum you can expect to pay for a simple DIY wood tank is 150-170. Not much cheaper if it is even cheaper than a store bought tank. One thing to note: a sheet of 4x8 plywood is just barely enough to build a 100 gal tank, if you choose to build a larger tank expect to buy two sheets.
I chose to build my tank because i loved the project. i am working on several other DIY projects to go along with the tank such as the stand, the hood, the background, sump, fluidized bed filter, moonlighting... I just work on things in my spare time as I go and have the money to invest in them. I hope this helps you decide. If you are really curious about building a tank of your own you may want to refer to this link:
Plywood Tank
Plywood Tank 2
Oh, a neat trick not found in either one of these articles. When installing your glass you'll find that the weight of the glass pushes out the majority of the silicon. Not Good. What i did, (found it somewhere on the web but can't seem to find it again) was use thin squares of neoprene as spacers. The stuff I used might have been between a 1/32nd of an inch to a 16th of an inch thick. I spaced it evenly around the area where my glass was going to sit (one spacer every 6 inches or so) then spread my silicon generously. When I placed my glass the silicon squished around a bit and gave me a great coverage area. It didn't get squeezed out because the neoprene acted as a spacer. The silicon surrounded the neoprene. I placed pretty much my entire library on top of the glass and let it cure for a week. One thing to remember. Silicon, as it cures, is very hazardous to your health. Do this in a WELL VENTILATED AREA. Once it has cured you are home free.. well with the glass anyway lol