endowment effect

You value items higher simply because you own them.

A 1990 study found participants were willing to pay $3.50 for a mug, but demanded $7.00 to part with it.

$2.50

Your attachment to what you have distorts your appraisal of what it is worth.

Where this comes from

Study: Kahneman, Knetsch, and Thaler (1990)
What the check found: The study and its conclusion are accurate, but the specific numbers cited for 'willingness to pay' are incorrect according to the primary source.
Confidence: Solid, with limits

One of these every day, and every one names the study it came from. When a finding is contested, it says so.