She didn't pay for either the dress or the ticket: The dress was loaned to her for the event, and the Met routinely gives complementary (free) tickets to elected government officials in New York whose constituencies include the Met.
All that aside, you think she was aware of the what she was doing? How stupid are you? This was 100% calculated to cause a sensation, which it did. I'm apolitical but I have to congratulate the shrewdness of the planning and execution.