If you can make one yourself for $30, why would anyone sell the thing ready made for the price of raw ingredients?
You don't usually find a pair of jeans for $ 3 even if it'd cost that to make them with the right tools, or be able to buy a bottle of vodka for the pure price of manufacturing it. If I made the tanks, I wouldn't sell them for the price of ingredients - time and effort are money.
Asking a number of diy-imacquarium owners should give some clues on where they got their kits or ingredients...