If it was in anyway goofy then car lenders and mortgage lenders would purposely not disclose that information.
Like I said you can go to any real estate website right now click on a house for sale and view the history by sale date and price. You don't even have to register. It's not some secret, it's not rude.
Just randomly asking someone how much they paid for something is rude. If you are on the subject it's not rude to inquire as long as you do it appropriately. If someone shows you their new motorcycle and you say "how much did that cost?" That's not rude. It's contextual, it's about how you broach the subject. One doesn't even have to be direct. You could be looking at someone's Corvette z06 and ask "how much do those sell for?" Rather than "how much did YOU pay?"