P.W.
Unless Catalina's changed a lot in the past few years - it's not worth $300. It's cute, and it's kind of fun to ride the ferry, but the last time I was there, there was nothing there except a small street and the Casino, but I don't even remember anything going on in the Casino. Of all the places you can spend $300 in So. Cal, I wouldn't spend it on Catalina.
They've probably added a few more cutesy shops, but I still don't think it's worth all that. It was a good scuba diving spot many years ago, so if you're going to scuba dive, then go.
Otherwise, I'd say no.
There is lots to do in San Diego, if you get that far.