I am running into the same pricing problems as well. I am trying to get a date and location set as far in advance as I can so that my guests can plan. At the rates given though, I don't think many of my guests will be able to stay at the all-inclusive hotels we are thinking of having the wedding at. I am thinking about reserving the wedding date and booking the hotel stay in a few months when prices go down. I'm not sure if this is the best way to go though.
Prices quoted by my TA for Dreams Tulum, Dreams Cancun, and Azul for June/July 2010 attached.
Cancun - Tulum.xls