When we were at the resort in 2009 we didn't bring much cash, probably $150 or so. We wished we had brought more as most of the vendors seemed to prefer US $. When we got pesos from the cash machine it was really hard to figure out the prices on things because the vendors quoted everything in USD, and I think they actually charged more when we used pesos. It was also easier for us to figure out how much we were being charged when we spent USD.
One of the nights at the resort is a fiesta night. Last time they did this, there were a bunch of vendors right at the resort out around the pool. They only took cash, we couldn't charge their stuff to our rooms. There also used to be a bunch of vendors that set up shop in a giant tent/palapa right next to the resort out on the beach. They also only take cash, and most prefer USD.
So, how much cash you bring will really depend on how much you plan to shop, and where you plan to shop. There is a mall up the street from the resort, and I think most of the stores there take credit cards, MC and Visa anyway (I think).
We like to shop, so we'll be bringing a lot more cash this time!
Staci and Amy