Try Flyaway: http://www.lawa.org/flyAwayInfo2.cfm
I've never used it, but I've heard about it (on this message board actually) and it seems like it'd be the cheapest option by far. From Union Station, you take the San Bernardino Line to Claremont (NOT Pomona). You'll be literally a block away from the campus where you get dropped off. Just double check with metrolinktrains.com to make sure there is a train leaving Union Station for Claremont by the time you get there.