Exactly! If you have spent time, as well money on petrol to get there, why would you want to head back without getting a feed? You might as well go!
Okay, so maybe the restaurant example was flawed.
The point is, whether you go through with it or not, you can't change the past. Therefore, it should not affect your future considerations.
In the example of the camp, it is irrational to consider the camp deposit as a factor in deciding on whether to go - because whether you decide to go or not, you will still have paid the camp deposit. You can "cancel" that "factor" out from both sides, if you will.
Suppose you like this girl, and you want to take her out to a movie. You pay $20 in total for the tickets and transport there. But she hates the movie - and is offering $15 worth of ice cream, her shout, for ditching the movie.
What do you do?
On the one hand, you might say "I paid $20 for the tickets, and you're only offering something worth $15. So I'll pass." That's what I think you're getting at. "I've already paid $20, so why leave? I might as well get my money's worth."
You have two options:
1. Not go because you want to get your money's worth. You lose $20.
2. Go. You lose $20, but get $15 in free ice cream. You lose $5.
But what if she offers $25 worth of ice cream?
1. Not go for some reason. You lose $20.
2. Go. You lose $20, but get $25 in free ice cream. You gain $5.
In both cases, it's more rational to go. The tickets are a sunk cost, you can "cancel" it out. Therefore if she offers you $15 of ice cream, the two options become:
1. Not go. Gain nothing.
2. Go. Gain $15.
If she offers you $25 of ice cream, the two options become:
1. Not go. Gain nothing.
2. Go. Gain $25.
See what I mean? You can't escape the fact that you will have already lost $20, so it shouldn't matter what you do. It can't be recuperated. What you
can do is make good decisions to minimise your loss, or maximise your gain
in the future.
I hate to say it but not many people apply the sunk cost principle in real life.
But they should.