Of course there is Cayo Espanto, on a small private island on the back side of Ambergris. It only runs about US$1500 to $2300 a day, plus hotel tax and 15% service, all-inclusive.
A few of the resorts on Ambergris will do semi-all inclusive packages, with meals and transport included. But as noted there's really nothing on the island similar to the all-inclusives in Jamaica or Mexico, which offer lodging, meals, booze and activities for a relatively low flat rate.
With all the great restaurants, bars and activities in San Pedro, it's easy to understand why Ambergris Caye isn't an all-inclusive type of destination.