Hi, Chris.
You have a few options:
1. Fly. UA Express (SkyWest) has 6 flights on Saturdays LAX-ONT. Often times UA has these on their "e-fare" specials at united.com for $90 round trip. This might be worthwhile if rental car prices at ONT are cheaper, or if you can get a free hotel shuttle from ONT airport. Plus on the return you would probably be able to check your luggage through to your final destination.
2. SuperShuttle. The website lists $48 one way from LAX to the 91761 zip code in Ontario. PrimeTime is the other major shuttle service but it has just temporarily shut down due to insurance issues.
3. Rail. It's a bit complicated but if you're not crunched for time, it's cheap:
a. Take the LAX Shuttle (free airport operated bus) from outside baggage claim to the MetroRail Green Line Aviation/LAX station.
b. Take MetroRail Green Line east to the Blue Line north to the Red Line east and get off at LA Union Station.
c. Transfer to Metrolink: the Riverside line stops in East Ontario, the San Bernardino line stops in Rancho Cucamonga. From there you'd be on your own in terms of calling a taxi, using local bus transit, or arranging for a rental car pick up. But the total price would be $9.50 one way. ($3 for a MetroRail day pass and $6.50 for the Metrolink.) More info on MetroRail is at
www.mta.net, Metrolink is
www.metrolinktrains.com