I'd think a simple "its not for sale now, but give me your number and if I change my mind, I'll call" would deflect a lot of them. Doesn't mean you have to call them. If it doesn't deter them, then hit them with insured value or the price of what your next desired purchase would be.
I want a '70 Road Runner, Air Grabber with a 440 6-pak and a four speed. Going price on those is $75k +, so that's what my Challenger is worth, despite the fact it has bondo, body rot, and needs paint. If someone whats to disagree with the value of my ride that isn't for sale, that is my logic of justification if I'm going to let it go. They can then chose to disagree with me, but if they want my car, they can cough it up or move on.