A more direct, less cheeky version would be to ask what they are picturing for their website and giving them some estimates.
If a car salesman were asked that question, he wouldn't answer it flippantly because he knows that's a little embarrassing to make the customer feel ignorant to that degree. You're invalidating their question, not answering it.
Instead, the car salesman would say, "That depends on the kind of car that fits your needs. What kind of vehicles are you thinking about?"
There's no harm in making the car analogy explicit, but I'd think prospects would be warmer to more helpful and direct responses at first.
