Many Choices.
She is a woman, her ability is an alpha, and she is beautiful. It was a trifecta of qualities designed to make her expensive. The fact that she can share her ability with others increased the price even further.
She can bestow her mental shields on others so she can be used as a bodyguard of sorts. This shields will be weaker and she can only bestow one on others, but it still made her worth a hefty 10 million dollars.
She was an expensive slave. If not for the fact that her ability was so strong, he wouldn't have considered her. There's others the fact that others would look down on her because she was unranked, so she would be a very weak bodyguard. This made her price still manageable.
Apart from power ratings, she was also not the one with the strongest mental resistance ability. There was a man who could reflect mental attacks back to the source. There was another woman who could devour mental attacks to replenish her focus.