You must be joking is an exclamation of disbelief. You must be joking, at that price.
It is used against a proposal rather than against news, so it usually refuses something. It is not a question and expects no answer.
1phraseyou must be jokingAn exclamation of disbelief.
“Walk it in this weather, you must be joking.”
you must be joking is a phrase.
Practised in Small Talk & Everyday Phrases