Vacation to Mexico in December: Price and where to Go?

[EvitaPeron]

I want to travel to Mexico in December. I have another friend there, that most likely will travel with me like a guide.
Where can you recommend me to go?
And how much do you think I would need to take with me if I will spend there 1 or 2 weeks (I know for ticket I will probably spend $300-$400).

I was thinking about going to Cancun, but what do you think other great destinations during December.

[not_exactly_a_lightweight]

stay out of my beach community