The ponchos in the parks aren't too outrageous, especially since they can make the difference between wet and happy and wet and miserable.
But when you KNOW the weather's gonna be wet, and you're going out in it anyway, ask the resort housekeepers for some of those plastic shower hats. You put on your socks, slip the shower hats over each foot, then put on your shoes (because you're wearing comfortable shoes to Disney, right?). Your shoes may not be waterproof, but your feet will be!
We take our wonderful, complimentary USA Today and stuff our wet shoes with newspaper at night, swapping it out until the shoes are dry. Plus, we always bring two pairs of shoes a piece. We've been at MK literally ankle-deep in water, and still managed to have a good time.
Hope you have great weather!