There is no easy and cheap way between San Diego and Magic Mountain. It is at least a 3 hour drive depending on traffic. That makes a taxi very expensive.
LAX is probably 30-45 minutes from Magic Mountain, again depending on traffic.
If your going to return to San Diego, rent a car. If it is one way, the cheapest is going to be the train. I think there is a stop by Magic Mountain.
Unless you want to spend a lot of time in San Diego, why don't you fly in to LAX and stay at a hotel in the LA area, From there, there are tour companies that will provide transportation and a ticket to all major attractions.
http://www.la-tours.com/