Concerning plastic pipe, there are reginal differences. What is cheap one place, can be expensive somewhere else. It depends on what the supply companies stock, which depends on building codes and demand. Take a look at say the 2 cheapest 12" pipes they have.

Also take a look around. For example plastic barrels are cheap in south Texas, old sprayer tanks can be had for free some places and one year it seemed every sailor had had nylon sail tubes when a local plant changed product lines.

If you are willing to spend $800, check the local aluminium tool box builders. For that much you can have a box that is really waterproof and will outlast 3 boats.

If you have ever thought about building a boat, an epoxy/glass/marine plywood box will last a long time and let you know if you are going to really hate building.