One of the cheaper source for aluminum sheets and bars is Aircraft Spruce, where you can buy it online (
Aluminum from Aircraft Spruce). They are MUCH cheaper than Lowe's or even McMaster-Carr.
One problem with using Coroplast for a trailer (or anything that will be continually exposed to the sun) is that will get brittle and start to break down after a year or two of sun exposure unless you keep it painted (and paint doesn't stick very well to polypropylene).