The word "massive" and "cheap" will never be. Ebay has 250gallon acrylic tanks for I beleive 900$ thats pretty much as cheep as you will get. I dont know what glass will cost, but doing a 8 foot plexi glass tank yourself will be a little cheaper
failing that you could go to pets at home they do the basic tank without fiklter stand and heater but ther quite cheap i got a 2.5ft tank from ther for 1 of my goldish for about £20 i would say they were the cheapest i foumd them thy have loads of diffrent size tanks up to a large size
altho i wouldent recommed going to big i would ay if you are making the stand youself only do about a 3ft tank because if you get anythink wrong when doing sommat like a 5ft tank the weight of the tank when filled could break the stand and leave you with a big mess, also if you are doing it in wood its not a good idea unless you get your own as school have contracts with dealerships and they basically get scrap wood dirt cheap of the dealers so normally the wood wont be as strong as usual i nkow this because i made a rocking chair and well the woodn seat brok in the middle as soon as someone sat in it altho he was a bit of a large bloke lol
...and as 1litre of water weighs 1kg, you're looking at 727kg + the weight of tank. A very substantial weight indeed, so take that into account when building your stand!
don't worry i intend to get my own wood, i know the school wood is very weak remember i going to spend a threw mouths designing it them few mouths building it.
whats wired is that the 6ft, 2.5ft, 2ft is alot more than the 8ft,2ft,2ft those anyone though why?