Not the story I was looking for, but similar:
The Tale of the Blacksmith and the Single Nail
A horseman arrived in a village and went to see the blacksmith because his horse had lost a horseshoe. He asked the blacksmith what it would cost to forge a single nail for the shoe.
"One penny," the blacksmith replied.
The horseman thought for a moment and then asked, "And what would ten nails cost?"
The blacksmith pondered this and said, "Ten nails will cost a silver coin." (A price significantly higher than ten pennies).
The horseman was confused. "That's illogical!" he exclaimed. "Why does each nail become more expensive when I take more of them? Usually, you get a discount for larger quantities!"
The old blacksmith wiped the sweat from his brow, looked at the horseman, and explained calmly:
"It is very simple to forge a single nail. I take a piece of iron I have on hand, heat it in the embers that are already glowing, and with a few strikes of the hammer, the nail is done. It hardly disrupts my workflow.
But if you want ten nails, I must interrupt my actual work. I have to select and cut new iron specifically for them. I have to stoke the fire in the forge, bring it to the perfect temperature, and maintain it. I must concentrate on forging ten identical nails of consistent quality. That costs me more time, more coal, more concentration, and it keeps me from larger, more profitable jobs. Making ten nails isn't just ten times the work of one nail—it's a separate, more demanding task."