Hyacinth: Is that bag from X designer?
Me: No Thank you. (said with a big smile and a cheery tone)
Then turn to something/someone else. This is my go-to answer for anything nosy. Well, when I'm on the ball anyway
This is a polite reply, so one one can accuse you of being rude. But it completely derails the other person. Unless Hyacinth is like the person who didn't even hear the reply of "the wall fell down" you can escape while she is tying to get back on track.
This also gets you out of having to do a lot a mental work - unless you like the idea of saying something cost a million dollars and a pink sparkly unicorn. If I had a pink sparkly unicorn I wouldn't sell it for ANYTHING.