I would like to find an all inclusive hotel in Cancun of moderate price. I would like to book a package with airfare as well. I will be traveling with a 13 year old and mother. I don't want to be at a hotel with tons of little kids running around, just a place to relax.

You don't say where you live. Check out www.applevacations.com and www.funjet.com.
Also check with Liberty Travel. All these places offer packages with decent prices for the Cancun area.
If I were you I'd check outside of Cancun in the Riviera Maya or Playa del Carmen area. You would still fly into Cancun.

Why Riviera Maya or Playa over Cancun?

check out Dreams Cancun.

Cancun is EXTREMELY commercialized. The Riviera Maya and Play del Carmen have more of a "Mexico" fell to them.

I agree! Cancun is like going to south Florida. You might as well stay in the good ole USA. Playa del Carmen has some commercialism and american touches, but is still a great town with Mayan character and charm. The beaches are beautiful and the town is wonderful. I would even suggest an non all inclusive as the restaurants in Playa are wonderful and a 13 year old is certainly not going to be taking advantage of all inclusive alcohol. There are some nice hotels. Many even include breakfast or 2 meals ie: Shangra La (neat palapa stlye places). Many that I've looked at are adult only...not sure of the age limit. La Tortuga, Mosquito Beach. If you want an AI, there is the sister resort to the Royal Playa del Carmen which is nice, but beach area is not as nice. Gran Porto Real (I think). I've heard decent things about Sandos resorts, but they are not within walking distance of all 5th st. has to offer.