We paid 660 Pesos last week, though they'll also take £38. US Dollars are not accepted from UK travelers.
Cost is per person and children pay the same, so it's a fair bit of a cash for a family of four.
We had conflicting info on airport cash machines, one rep thought there were three, while another reckoned there's only one which regularly runs out, so as juby5 says, best to get it before hand.