Anyone trying to hide how much they paid for a house is probably trying to hide something else. After all, real estate transactions are genrally matters of public record. In fact, all I have to do is punch in my street address - not even the city or state - into google, and the very fist listing is from the Detroit Free Press listing the sale price (they got it slightly wrong, but there it is...).
I might preface it with a "If you don't mind me asking", but it really isn't that hard to find out.
Asking about the mortgage they took out on it, however....