bill your dead on, used tanks are dirt cheap. i just picked up a 30gal w/ cast iron stand, hood, lighting, for 30 bucks. even though i am only using the tank itself it was a **** good deal.
a few months back.. maybe in august some guy i knew through another forum was willing to give away a 150 gal
, 75 and 10 gallon, complete systems (stands, hood, heaters, filter, etc even the fish and rock) if someone would just take them by a certain date.
point i was trying to make, is check garage sales, classified ads from your local newspapers, flea markets, etc you would be surprised to learn what people are willing to take for used used tanks.
at best a diy
glass tank will be about 5-10% cheaper than buying a manufactored tank. but the problem with diy
'ing a tank is finding a glass supplier, (who might have tempered glass to match your size requirements) making sure the corners are cut perfectly, then during construction the panes are in perfect alligment, and that its totally level. if everything isn't perfect your diy
tank will suffer from minor failure (leaking from joints) or complete failure where the pane of glass snaps in 2 pieces or shatters.
a plywood tank - is made from mostly plywood, where the bottom, sides, and back are made from sheets of plywood, and the front is a mix of ply and glass. the plywood is painted and sealed, so its hard to tell what was used to construct it, and these only tend to be 5% cheaper (if that anymore) than manufactored tanks of identical sizes. you can try searching there are dozens of threads asking your same questions, many with links to other threads of users who have built plywood tanks (mr. burns comes to mind)
acrylic is still more expensive than glass, even though its easier to work with so you won't save much (if anything) if you choose the acrylic route.diy
tanks should only be thought of, when you want a tank to fit into a certain area a commerical tank won't fit into and have a certain gallon capacity, or when you have an unusual idea for a tank shape, trying to save money by building your own will get you very limited savings or cost you more than a manufactored tank - we all make mistakes
i highly suggest you start shopping around your classifieds and flea markets and garage sales.