The towncar services run anywhere form around $80 - $100 round trip per car. I don't know what a taxi would cost, but since the taxi has a meter you would pay extra for any traffic delays.
The cheapest way for one or two people would be the Mears shuttle. However, they stop at numerous hotels along the way and it sometimes can take a couple of hours from the airport to your hotel.
I just noticed that you have 4 people so definitely use a town car service. It will be cheaper than Mears and a lot more comfortable. Keep reading these boards as there are many different services available.