i dont know where your looking but all the yz450f's ive seen that havent been raced are running 5,000 or more, and the raced 1's are still at like 4,500.... i did however find a 2006 kx450f race bike for 3,800.... but anyways do you mind me asking where your looking for these bikes?
Usually craigslist, ebay, etc. The main thing that drives the price down is the price of a new bike that the dealers are trying to get rid of. Like the Vickery price, I mentioned earlier, of 5499 for an 07 really makes a used 06 worth less. For waht it's worth, last winter I tried to sell my 06 and the best offer I got was 4700, It had 2 rides on it. And, I only got 2 calls on it. I priced it at 5000 obo. People can ask whatever they want, but that does not mean they will even get a call. For example, there is a guy, craigslist in denver, that says 7000 firm on an 06 with some goodies on it. He won't get a seious call on it. But, his add give me a good laugh.