We're a specialty coffee shop, so we're going to charge per gram of coffee brewed... not by how much water ended up in your cup.
"You'd like a coffee - specialty grade? Excellent. We're serving a Colombian from XYZ estate, roasted by ABC. It's costing us 9.5 sen/gram, and the dosage right now is 20 grams, but it's still a bit too fresh and boring, so the markup is only 3x today - we recommend it black. Black is fine? Ok. Five-seventy, please. Milk on the side? Really? One ringgit for a once ounce serving, only. Thank you very much, here's your change. Come back in two days - we'll probably be charging 5x but it'll be worth it, if the previous batch is anything to go by..."