This strikes me as both rude and bullying - sheez, they ganged up on you! - so my guess is that it comes from jealousy. Why should they care how much you spend on your clothing? Is it any of their business in any way?
I always try to remember Miss Manners in these situations. She often said that most people write her saying someone was rude, and what is the politest way to tell them to go burn? Which, as she pointed out, is not the point of manners at all.
Along with that, I really hate the way women default to putting themselves down. The "oh, I suck really and my clothes are really cheap" response may be true, or it may not be, but honestly why do we do this?
So, in thinking of both Miss Manners and the need for personal and professional pride, perhaps...
"Oh, I never discuss how much things cost."
Or
"I enjoy looking nice."
Or, if you want to be confrontational, try,
"Why do you care what I wear?"
Or
"Pardon me, why is this important...?"