"Cheap is easy in Mexico" True. Very True.
The most expensive part will be getting there. The best airfares are through discounters or junket bookers.
All Inclusive (AI) are generally only a good 'deal' if you plan to just hang out on property and lounge and eat and drink.
If you want to fly to Cancun and then travel around you can take Clavus' sage advice.
Do you speak Spanish? IMHO you would almost have to do the travelling around plan.
BTW, January is high season.
Try this website. This is a cool place. Not overrun with tourists and really a slice of Mexico. Check out the message board.
http://www.isla-mujeres.net/home.htm
Good Luck.