If cost is an issue and you need to do something right away with the rotting floor, then your're right, use your PT plywood. I know others in this forum are going to chastise such advise but I understand YOUR dilemma. So let em rip!
Hopefully the porch is covered. That will at least help in extending the new floors' life. Also see if you can get it T&G (tongue and groove). That will help the seam problem.

Remember to paint all the plwood edges. They are like sponges that absorb water.

Here is a link that might be useful: my album

PT plywood will react the same way any plywood will exposed to the weather.T&G material will make the problem worst.

If a cheep fix is what your after use regular cdx. J.