Don't ask how much someone paid for something -- especially a car or a home.
Don't ask how much someone makes, or the amount of their last raise, etc.
Tying back to this, don't make someone feel bad for what they paid for something. About 5 years ago, a lovely woman moved into my condo building. The association president introduced herself to the new neighbor, and said, "I saw what you paid. You overpaid!"
What could you possibly gain from saying such a thing?
Easy etiquette rule: When it comes to money, keep your mouth shut.