Getting to Tijuana airport even from downtown takes about two to three hours. Then on top of that transit time, you need to arrive there about two hours early. So going from the San Diego airport you’re wasting four hours just one way!
So to get to the airport will cost gas and cab money and possibly bus money which is about $50 one way. Both ways it will be about $100. Then when you arrive in Tijuana, as a non-Mexican national you have to pay to fly and that is about $40. So in the end you will waste eight hours and $140 which is probably how much it would have cost to fly from San Diego Airport except you don’t waste the eight hours.