I find that jokes tend to diffuse most odd situations. My favorite response to this one is to laugh and say "God, more than I like to admit..."
Usually that gets a laugh and ends the conversation. If they persist, I tell them honestly and say "Why, were you looking to get into photography?"
I've had a few pleasant conversations from this.
Frankly, if people really want to know how much my camera is, what the hell do I care? Sure, I'll tell ya. Some people marvel at it, some people are horrified, some are disgusted that I would be stupid enough to do such a thing. <shrug> All of those reactions are pretty amusing, and it was they who were rude/silly/brave enough to ask, so no skin off my back.
EDIT: Further frankly, I'm proud that I can say that I have paid for all my equipment... with my equipment. This makes people's reactions even more entertaining at times. Esp. the ones who think I'm insane or stupid for spending $1800 on a camera.