In article ,
Mark Anderson wrote:

I have some exterior plywood that has been outside for twenty years nd
it is still going strong. It would be better if it had a good paint job
but plywood is superior to many other kinds of wood. It is hard to beat
the price of plywood.

Dick

As someone else pointed out, you can get a 4x8 sheet of birch plywood
(3/4") for around $30 (in my area). That stuff is pretty excellent wood
and even cheaper than pine boards but you'll need to either paint it or
treat it yourself to keep it from succumbing to moisture.