On what did Mildred want to spend the money?
I do not know for sure what you mean by "the money," but I think that what you are thinking of is that Mildred Montag wants to spend money on getting a fourth wall for their parlour.
Mildred is totally into the "families" that she watches on the parlour walls. Just two months ago she had a third wall of their parlour made into a TV wall and now she wants a fourth. She tells Guy that it will only cost $2000. He points out that that amount comes out to one third of what he makes in a year. She doesn't think that it's really all that much money and she thinks he ought to be more considerate of her needs.