In this day and age of the internet and dealership websites, Autotrader etc, I think most people go to a dealership knowing the exact car they want to look at, so a salesman asking how much you have to spend isn't relevant as you know the car you're after. Surely the days of walking into the local 2nd hand dealer not having a clue what you're after are long gone?
I know they still ask about your budget, but I never tell them as it's not important, I'm here to look at this 1 car, not their whole fleet.