Luna wanted to be offended by what Mina had said about her family's generosity, but it was true. Neither she, nor her brother, did anything without reason, and that reason could be anything from self-preservation to putting in for a favor to cash in on later.
It seemed that for Mina, the dress that Cylus had given her was the cost for asking Ian for help. Luna had seen the repulse on her brother's face when he tossed the beautiful gown to the floor like it was nothing. While to him it was just a dress, Luna knew at first glance that the pale golden fabric was genuine silk, and the embellishments were likely pure crystal.
It was a dress fit for only the highest of the upper class families, and Luna could tell that Cylus, being a man from the entertainment circle, knew exactly what message he was sending by giving Mina such a gift. He was telling her that to him, she was a princess, while in stark contrast, her brother treated Mina as if she were a servant.