It all depends on how the bike/engine was maintained. For example, I am currently selling my 08 Hayabusa with 32,xxx miles on it for $8,000. That sort of mileage may make people shy away from the purchase, but 90% of those miles are highway at 3500 RPM. All maintenance has been performed with top notch parts. You can generally know how well a bike has been taken care of by giving it a close inspection and asking the right questions. If you ask about a valve adjustment, and the seller blows off the question or answers it with an awkward answer, then you should be cautious. Some of us use our Hayabusa for touring/commuting and it is not uncommon to see bikes over 50,000 miles with zero problems.
If you are looking at a "high mileage" bike, then be sure about the quality and consistency of the maintenance and the knowledge of the seller.