You can definitely squeeze the family into one taxi (which can hold 4 adults w/ 3 squeezed in the back)....of course all your luggage will have to fit in the trunk....or sometimes there are roof carriers. I wouldn't let myself be double charged for a van.
Taxis charge by zone, and luckily you are closer to the airport than many places, so $15 US should be about right......If you dn't have pesos, there is an exchange at the airport which isn't too bad a conversion. Whatever the charge going, it will be only 60% of that price for the return.