We recently traveled to DisneyWorld on the Prince and Princess Plan. I can't remember the exact amount, but I think it was about an extra $100.00 for the plan.
We used our 50 Disney Dollars, character breakfast coupons (worth $52), $25 discount at Planet Hollywood and 2 Disney pins and 2 lanyard (~$20). This was probably worth a little more then the amout that we paid for the plan.
We did not use the special seating coupons. These coupons required either a PS for each show or arriving 30 minutes before the show. We were able to get good seats by just walking in prior to the show.
The special seating coupons are valuable only if the park is very busy or if your prince or princess would like to participate in the show.
-Paul