really hard to predict. I would search around for online menus for your hotel and the restaurants near it, just to get a feel for typical hotel priced food. You might have to find mall restaurants for online prices. A site like Chowhound.com might have some links to restaurants with sites so you can get a feel for prices.
there are cheap buses for getting around Cancun (to downtown, or to the ferry for Isla Mujeres) - you dont have to take an expensive tour for everything. I highly recommend bringing the family to Chichen Itza or another closer ruin, regardless of the cost - it will be worth every penny. Cabs are also relatively inexpensive - just be sure to ask before the ride starts about the cost.
and, remember, 15% tips are typical for sit down meals in restaurants (not necessary at a taco stand or take out place). $1 per drink tip at a bar is acceptable. Daily tip for the housekeeper and anyone who provides services like bringing your luggage to your room.
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