The phrase 'rip off' suggests a dishonest act, expressing disapproval and negative feelings towards the action. Charging double the price feels like a rip off.
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Informal Usage
'Rip off' is informal and often used in casual speaking or writing, not in formal situations. Don't go to that shop; it's a rip off.
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Strong emotion
It often indicates frustration or anger when realizing one has been cheated. I felt so angry realizing the ticket was a rip off.