"It's what the evening costs"

Here's a belief that arrives as arithmetic. An evening out costs sixty-eight pounds. Lisbon costs four hundred. That's the price, and the only question is whether you'll pay it.

The price of an evening isn't a fact about evenings. It's a decision, made by whoever chose the restaurant, and then adjusted by whoever ordered the second bottle, and then finalised by whoever said "shall we just split it?". At each step, a person made a choice, and you weren't that person, and you paid for it as though it were the weather.

The same six friends could have had an evening for thirty pounds each, at a different place, with the same conversation. The same hen party could have had the weekend in a cottage two hours away for a third of the cost. The conversation, which is what you were paying for, costs the same at every price point. What changes with the price is the setting, and the setting was chosen by the organiser, who chose it for reasons that had nothing to do with your month.

The split is where this belief does its most expensive work. "Shall we just split it?" is a sentence that turns a decision, whether to pay for what you had, into a courtesy, and declining a courtesy feels churlish in a way that declining a bill doesn't. But the split is a transfer. It moves money from the people who ordered less to the people who ordered more, and it does so silently, and it does so to the people at the table who are most likely to have ordered less because they were watching the price. If you had a main and a glass and paid sixty-eight, thirty-three pounds of that was a gift to the far end of the table, made without your deciding to make it.

So the price of the evening is a choice, and it's usually somebody else's, and the method in chapter 20 is to ask before the choice is made, when it's still a question. "Where are we thinking? Roughly what's it likely to come to?" Once you know the price, you can decide whether to pay it. Before you know it, you've already paid.