The best rate you'll get is from your bank. That's will give you interbank exchange rate + 1% which will be better than any money exchange place you can find anywhere. You can either do it before you leave the US or if you can somehow get the non network ATM withdraw fees waived, then you can either take the peso out from the ATM down in Cancun. For example, if you have Bank of America, there is no non network ATM withdraw fees if you use Santander bank ATM in Mexico.
I agree, I think it depends on your spending habits. I'm going for 5 days and I'm taking about $1500. But I dont have all inclusive and I drink a lot, and I have a few day trips planned.
we are going for 2 weeks and bringing 1500 each..... but we will probably only go to the clubs less than 4 times! no big clubbers.... although i loveeeee to dance! We will use it mainly on all the tours and junkkkk