Lock the door, drill the core. Charge them for a new core + re keying + installation and wear on the drillbit. Use 30 minutes and make sure the price is 3x of your regular unlock fee. Now they are 6x on your time for only 3x the price. Super bargain
That reminds me of the joke where a factory has a big machine break down. They call in a specialist to fix it.
The specialist looks at the machine for a moment, hits it with a small hammer and it starts working instantly.
But on being told that the repair cost is $500, the factory owner is outraged and asks how that can possibly be justified for less than a minute's work.
"Well, it's $5 for use of the hammer, $495 for knowing exactly where to hit the machine."