Evident by the number of varying answers here, you certainly have a lot of directions you could go with a response. Whatever you choose to say should take into account two related but distinct considerations.
- Suit your personality
- Factor in what we in the community organizing community would call your "organizational considerations" – think of this as how you want to leave things with the person asking.
The first part is easy: if you're a smart aleck, you could throw out some witty, flippant, or humorously evasive answer (see numerous possibilities above); if you're more the brooding or quiet type, you could be more direct about how you feel it's inappropriate for them to ask; etc. I don't know your personality beyond your wording of the question and various comments throughout the page, but what I can guess of your personality is factored into my answer below.
The second point is probably due more consideration on your part. Can the person in question be of help to you in the future or do you have other reasons to not respond in a way that might otherwise, based on your personality alone, be flippant or dismissive? As the question is likely asked by numerous different people, with whom you have numerous kinds of relationships (and so, you'll have to figure this out every time), I cannot really go much further. Please advise in comments if you need more clarification about how this works, but I think you get the idea.
In any case, I advise the following, to be adjusted as need by the two points above:
To be honest, it's probably less than you'd expect. Beyond that, with respect, I would rather not talk specifics.
You could add that ranges can easily be found online and maybe specify your specialty or some field terminology to give them a nudge in the right direction – querying Dr. Google can be challenging without specifics. And if your answer feels like it needs some upbeat follow-up, you could add, "but if you want to talk tech, I could go around with you for hours!"