Park at the Solana Beach train station for free, pay $11 round-trip on the Coaster. Make the connection on the trolley if you don’t want to walk from the station downtown (but it’s not that far).
If your conference runs later than the last north-bound Coaster, I suspect you could just use the trolley directly and park for free elsewhere (e.g. parking for free at the Q for Padres games).
