Why Does Everyone Want to Know What You Paid for Things?
Humans are psychologically wired to size up their own standing by measuring it against yours. People push for the exact number because of: 1) social comparison theory, they need a benchmark to know where they stand 2) information asymmetry, whoever has the number has leverage in the next deal or ask 3) plain old status anxiety dressed up as friendly curiosity. You don't owe anyone a number that only helps them and never helps you.
What's actually happening
Someone asks what you paid, flat out
You feel weirdly obligated to answer
That number becomes leverage in future asks
Redirecting the question protects your next negotiation
What you actually owe them
Nobody ever asks why they actually need to know.