Oh and $1 per gallon is out to lunch. My 200 has cost me about a grand so far, and it's not even got glass in it yet.
Maybe if you dig a hole in the ground and put a tarp in it, it will cost under $1 per gallon, but small things REALLY add up.... the lumber for my tank and stand alone cost $200. glass $300. so just to make the basic SHAPE of the tank alone is like $2.50 per gallon or more. Consider the pumps, heaters, filter media, test kits, gravel (gravel is PRICEY if you're picky), sponges, screws, glue (I probably spent $50 between screws and glue), sealant (epoxy/drylok, whatever), silicone, FITTINGS (you'll need approximately 10,000), Pipe, PVC cement, paint brushes, rollers, mixing buckets, paint trays, rags, solvents, light fixtures, light bulbs, wiring... the list continues, but you're probably bored by now.
I'd say $3 per gallon for a functional, but fugly tank. $5 or more if you're going to make an effort to do things nicely, hide your wiring and plumbing, etc. Unless of course you get a large number of your materials for free, or are willing to wait/hunt down bargains. Then you might get $1.50 per gallon, but it will take 10 years to build.