"She didn't sell it for the money. She would have shown her evidence if she really wanted a big price. She sold it for the fun of it. For excitement." (10.13)
At the end of the day though, money isn't everything. In fact, for Mrs. Frankweiler, she can toss around the money however she wants—she's practically bathing in the green. She might as well auction off the statue for a loss so that she can laugh at all the idiots who didn't know how much it was worth.