Well, you can buy vinyl or PVC fabric for pretty cheap without destroying a perfectly good dress.
Here's one place that has it:
http://www.syfabrics.com/View.aspx/Clothing-PVC/White-Off-White-PVC/866/224
http://www.syfabrics.com/View.aspx/Fetish-Wet-Stretch-Vinyl-PVC/White-Stretch-Vinyl/599/225