L52 is usually competitive with SBP. I buy my gas on the west side self serve @ SBP. The folks at the FBO there are very nice. San Luis Jet Center on the east side are mostly catering to biz jets.
At the larger airports, like John Wayne, it is a matter of what the market will bear, I imagine airport fees and taxes. At my airport, the price is based on the cost of the last fuel delivery, so the price does not change until the next delivery, up or down.
At least in California, most of the City and County owned airports operate on what is known as an enterprise system. That means they do not get general fund money so they have to break even. The two main forms of income are hangar rents and fuel. In our case, they keep raising the hangar rents to cover their beauracracy.
Gillespie is county owned and is more than likely an enterprise department.
If you decide not to buy gas at your base airport, they will find another way to cover their costs and it will not include any cost savings on their part. Most of them know nothing about business. So don't expect them to make any kind of competitive business decision.
"Never feel sorry for a man who owns an airplane" Charles Morse (Anthony Hopkins) The Edge