Mental Accounting
Mental accounting is Richard Thaler's finding that people sort money into separate mental categories and judge each transaction inside its own category rather than against everything they own. It is why the same twenty pounds is trivial in one setting and worth an argument in another. It also explains why abandoning something you paid for feels like a loss: the spending opened an account, and walking away is what closes it at a loss.
What it is
An economist's version of a person has one pot of money and compares every possible use of it against every other. A real person has a holiday fund, a grocery budget, an account labelled "this evening", and a set of rules about which pot pays for what.
Thaler's second idea in the same paper is nearly as useful. Transaction utility is the pleasure of the deal itself, and it exists separately from the value of the thing bought. A marked-down grill delivers two things: a grill, and the satisfaction of the mark-down. That is why an advertised "expected price" announced before the real one changes how the real one feels, without changing the object at all.