You need the 10lb tank because it takes a lot of co2 to cool an intercooler. Have you ever put a peice of metal in a freezer and waited to see how long it took to get to the same temp as the inside of the freezer? How about with hot air going through the inside of the piece of metal?
It doesn't matter how much you can get a tank for, or how much you can get some hoses for. Anyone can make a diy co2 spray for pretty cheap, but if they can't get co2 for free, they're gonna end up spending a lot of money on co2. The co2 would have to be less then a quarter of the price of n2o to make it anywhere near worth what an n2o kit can do. This is cause you'd have to dump a lot more gas outside an intercooler then inside to have the same cooling effect on the air inside the intercooler, and the extra oxygen from n2o injection would help net more power while using much much much less gas. Alc/water injection has a much lower upkeep then n2o injection or co2 spray, but it requires a tune to get the results.