There is a new, fully insured shuttle service that will be starting up soon in Lone Pine. Call Bob at (760) 876-1915 to discuss your requirements.
One way to save some money is to take a shuttle from the Portal to Mammoth and take the YARTS bus from there. The YARTS bus is cheap (it looks cheaper to buy a round trip ticket and use just the first part than to buy a one way ticket, see the schedule at
http://www.yarts.com/schedule/2007/Summer120E.pdf . The limitation is that the YARTS bus leaves at 7 A.M. sharp. You would have to have the shuttle pick you up at the Portal at about 4:45 A.M. to be sure to arrive in Mammoth by a little before 7:00 and have a few minutes to spare. Or, you could have the shuttle take you to Mammoth the evening before and find the least expensive accomodation for the night.
Depending on your over-all time schedule and destination, you might be able to save a little bit by having the shuttle take you to one of the car rentals (Dollar Car and Enterprise) in Bishop. They do not appear to offer one-way rentals, but Enterprise might let you drop off a car at their office at the Fresno Airport if you have a second driver with you on the trip. It may be worth a couple of phone calls to see what can be worked out.
RWE