Having been guiding in Potrero for 13 seasons we go back and forth a lot from California to Potrero. We have found the least expensive option to be flying out of Tijuana into Monterrey. It sounds like it would be too much hassle but the Cross Border Express is a bridge that brings you from San Ysidro (just south of San Diego) right into the Tijuana airport.
The parking is the only problem if you are going for an extended trip. The least expensive option besides leaving a car with a friend that we have found is Lion Parking www.parkinglion.com which is $11 a day and they have a free shuttle to Cross Border Xpress Bridge (CBX).
Once at CBX you need a ticket. You can buy them ahead at https://www.crossborderxpress.com/ or when you arrive. They are from $20-$27 each way depending on the season. You will also need your boarding passes for your Tijuana flight, valid passports, and your Mexico immigration paper work you can print and fill out here: https://www.inm.gob.mx/fmme/publico/en/solicitud.html
It's a little complicated as you need to make an account on their site to make the payment. You need both the proof of payment printed as well as the immigration form. You can do it all at CBX and they have computers and people to help with the forms inside.
Once you cross the border and get through customs you pop right into the Tijuana airport and are ready to get on a $30 usd one way flight to limestone multi pitch paradise. I've flown through Tijuana many times over the years and the airport security there is the most strict I have ever seen for not allowing climbing gear on carry-on luggage. Years ago they wanted to take my rope so I went back and checked my rope. Then upon going through the security the second time they confiscated all my climbing slings and cordolette. Recently I also got busted with some climbing bolts and also went back and checked them in a separate bag. So don't let a $30 flight cost you $300 in climbing gear just check your bag through.
My last budget option for getting to El Potrero Chico from our place in the Eastern Sierra of California is flying from Las Vegas. While it doesn't have the $30 flights of Tijuana, they are still highly discounted and have direct flights to Monterrey. Vivaaerobus has non-stop flights we have taken many times. The best part of flying out of Vegas for us is the free parking from the RTC South Strip Transit Terminal. I have left my car for over a month and they have security on Segways keeping an eye on your car. They want you to contact them if you are staying more than seven days. Their website is here: https://www.rtcsnv.com/ways-to-travel/schedules-maps/park-ride/
Hopefully somebody finds this useful one day on the internet. I know it took me a few trips to get our CBX experience dialed, but now we have cheap inter-Mexico flights a short drive away and have been able to visit El Potrero multiple times in a winter without breaking the bank.