It's so rare I can accurately answer a question! Just keep in mind that the prices have changed, so this answer isn't entirely accurate.
When I went to WDW in September, I decided to do the Bounceback, since I was only there for 3 days and decided to go to a park a day. I paid $139 for 3 days, which was a savings of about $40 over the 4-day Parkhopper pass. It was also cheaper than the Length of Stay pass. I think it was Full Price for the first day, 10% off of the second day and 15% off the third day (doing the math, that seems about right). The trick is that (1) you can't switch parks at night, and (2) you have to pay for all the days at the same time.
I think the information would be on Deb Willis' site (www.wdwig.com).
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Donna