Maybe I'm just lucky, but I haven't had people asking me what I paid for something just to be nosy or disapproving. If anyone did, I'd use a PP's idea and say something like "A million bucks. The sticker price was $2 million, so I got a great deal!"
I'm more likely to volunteer the information if it's a really great deal, like the gorgeous, like new Ralph Lauren wool overcoat I got at the Salvation Army a few weeks ago -- 20 bucks! Not to brag about my mad bargain hunting skillz but to share how lucky I was to find it.
I've been known to ask how much something cost but with many disclaimers: "I know it's not nice to ask this, and the only reason I'm asking is that I've been wanting one, too, so if you don't mind, may I ask how much it cost?"
I got my first Mac computer this way, when a co-worker told me how excited she was about getting a Mac from a company that was converting to PCs and selling off all its Macs. In that case, it wasn't so much "What did you pay?" as "How much are they selling them for?"