I did it a few years back. Don't remember the cab fare from the place in downtown SD to the downtown train station, but I negotiated a flat rate with the taxi driver from the Anaheim station (at Angels stadium's parking lot) to DLH. Was something like $10 (meter showed $12) at the time.
I got to the train station early and was able to get on an earlier train than I had booked, which was nice. Shared the train with businesspeople and private High School aged students in their uniforms. Was a great way to get up to Anaheim.