$80 plus tip for most towncar services. ( Not sure about the limos). We usually budget $100 for a RT. That includes enough for a generous tip if the service warrants it.
It is a lot of $$ for one person and more expensive for 2 people than the Mears shuttle. When my 19 year old DD went with her 18 year old girlfriend (graduation trip), my DH & I arranged and paid for a towncar. It gave us peace of mind (parents always worry about their children, right?) and the girls felt very special/pampered. To us it was well worth it.
Hope something works out for you. Enjoy the trip!