I am assuming you are going All Inclusive, but the trips are quite good, if you can, do the Chichen Itza one, it is a long journey by air conditioned coach with nothing to see on the say cos it is pretty much all thru the low trees and a very straight road, but there is a stop on the way, don't but souvenires there. Take plenty of bottkled water as it gets hot at the Pyramid, do the guided tour, if u have kids though don't go here it isn't a kids thing. the journey is 2.5 hrs there and back,
The pirate ship night cruise is good and so are the clubs, Coco Bongo is excellent. The shopping Mall is great but not really cheap..
The chichen itza tour is about �50 per person if I remember but well worth it if u like historical stuff, it is amazing.
I would take about �250 each for the tours and excursions and try to get to Xaret too,
Then the souvenirs are cheap and cheerful so barter in the market, there are stalls outside Chichen Itza but they are really aggressive the stallholders and stuff is dear there.
The drinks in the clubs are not too dear but don't take a shot from the girls and guys blowing the whistles they ask for money afterwards lol
I took �800 for two of us for two weeks and I had plenty