It depends on how they ask, sometimes people ask and they come across as asking in a kind polite manner because they are considering purchasing themselves in which case I will tell them. I know I have asked people once or twice before I realized how rude it sounded if I did not know the person well. Other people may want to ask for the same reason but they just come across as nosy and critical in which case I don't want to tell them.... which brings me to a past experience I had with one such person....
This was in college and I has just met two people (a guy and a girl) and we decided to work on the midterm review questions together as it would be a TON of work for one person to do alone. I had my new purse with me and she started and asked me where I had purchased. I told her online and she then kept bugging me about what website. Then she preceeded to ask how much.... I just said a lot. But no... that didnt shut her up... she just kept guessing and guessing and I felt it was rude to not reply at first so I said 'more, more'. Finally I got so fed up with her guessing (she just wouldn't stop despite my obvious discomfort!!) and said 'I'm not saying anymore." She went on to criticize me for spending so much on a bag and somehow got the guy into it too. Both oozed a "OMG! You are SO materialistic for spending so much on a bag"-vibe to me. =T GRRR.. but then guess what? I'd say 4 months later I saw her at the bookstore. I know she saw me but it was odd that seemed to want to avoid me. I got a closer look and lo and behold SHE HAD GOTTEN THE SAME BAG!!!!!!!!!! ARGHHHH!!