I strongly encourage you to making the cage less than 10' long. Lumber almost always comes in 8' increments and you'll save money by limiting your maximum dimension to that. If you can I'd plan on limiting the floor area to 4' x 8'. As far as the height you should make sure it fits through doors. I'm sure you already thought of that but I usually try to point that out.
The problem with building the cages shorter is that it makes it a lot harder to get in and clean the back of the cage.
Otherwise, I think you can build your cage for less than $350. I recommend plywood over melamine simply because I think it works better for large cages and it's easier to seal with paint, epoxy, plastic sheet, etc.
I do recommend you ask this on the burmese forum and maybe even the reticulated forum. They can indicate what materials have held up well to the near corrosive effects of burmese python urates.