Putting aside the fact that they are "in-laws," I find that what appears sometimes to be "nosiness" is simply curiosity and/or being overly interested in your business.
It's not an easy thing to deal with and can be extremely annoying if you happen to be a private person in general, which I AM.
I'm a firm believer in straightforwardness in most personal matters. If one feels put out or really uncomfortable with their in-laws nosy behavior, say something. This can be done respectfully and diplomatically and more importantly, with back-up from your spouse (their adult child). Something to the effect..."I hope you understand, but I'd prefer to maintain what little privacy we have, by not discussing our personal & private matters with anyone."
You might also use the tried and true.......turning a question right back at the person.....If asked what you paid for your new car......simply smile and ask, "You know, probably around the same that you paid for yours. Car prices are fairly standard, don't you think?" (just an example) Most people would get the real message here that you do not intend to TELL them what you paid.
There are any number of creative ways to bypass nosy questions or nosiness in general, without hurting feelings or causing a family problem. Use your imagination and remain calm. One day, YOU will be an in-law too! LOL