The cost for an adult and an 11 year old are the same. The price depends on the time of year you are going. Likely your cost would be 41.99 per day per person.
Your post doesn't give details but guessing that you are choosing between the 40% off deal staying at the Polynesian or Free Dining and you have one or two adults and an 11 year old, it will be cheaper to take the 40% off discount.
A garden view room at the Poly is about 360$/day so 40% off is 144$ per day before taxes (I believe it is around 6.5%). The DP for 3 "adults" is 126$ per day (no tax is charged).
You can easily compare exact prices by pricing out both vacations on Disney's website booking from their "special offer" page.