Hahah 差不多 is always an excellent answer. When I first came here, taxi drivers would always ask me about salary. I tried telling them the truth, but then they would stop talking and I actually got ripped off a couple times. Then I would say a low amount but now that I can speak Chinese fairly well I just tell them that's a rude question in my culture. I like your idea of saying it's a secret. That will be my go to from now on.