You can check out supershuttle.com or airportbus.com for bus/shuttle rates.
When I was checking rates airportbus was the cheapest and we had a pleasent experiance w/ them last March. Taxi rates were about 75.00 ONE way from LAX when I called a few taxi services. The bus was about 35.00 round trip for my small family, so we went w/ airport bus. Super shuttle was about twice that price.
Make sure you find out when the shuttle/bus picks up from your airlines. AIrport bus picked up every hour on the hour and our flight arrived at 1:35 pm so we were able to catch the 2pm bus, but the family before us arrived at the airport at 12:55 so they missed the 1pm bus and had to wait for the 2pm bus. They were really frustrated waiting all that time inhaling the car exhaust.
If we had to wait that long, I might have spent more for supershuttle.... which seemed to be all over the airport.