Sounds like a pretty rude question to ask out right, but apparently the-auction-site-whose-mention-wakes-up-Peter-Reader has developed an app that lets you easily browse the sale prices of homes from all over the place.
Mine won't be on there, its never been sold, but I'm tempted to have a snoop around and see what the neighbours paid for theirs.
House prices are public information, I assume for historic reasons relating to rating, but they are just about the only thing that is. People can take an educated guess at what you paid for anything else - your car, your holiday, your business suit, but they can't just go online and find out down to the last dollar.
ETA the link I forgot :-)