at (one's) expense
1. One's financial responsibility. Once I learned that the trip was at my expense, I decided not to go.
2. To one's detriment or disadvantage. I was furious when I heard the other kids telling jokes at my little brother's expense.
at someone's expense
COMMON If someone laughs or makes a joke at your expense, they laugh or make a joke about you. Danny told them about my accident and they all had a good laugh at my expense. There was plenty of laughter, most of it at my expense.