my own experience is that prices can go down to roughly 65% of their value(current IL96 situation, not many companies buy them), or up to
maybe 150%. Ergo my IL96 advice : while it's silly to buy a IL96 at 100M$ when the 777 costs only 150M$, buying one at 66M$ when the 777 is at 220M$ is much more attractive. The move can be done quickly. In the current GW3, ERJ145STD's price went from 20.5M$ to 13.5M$ in less than two years - I was nearly the only customer then. New airframes of the same family, like the 145ER, didn't see their prices go down, for now.
Of course, a company that flies already 11 777, already ordered 32 more, and has a main base at Guam(far from everything, a good part of Europe & USA is beyond 6580NM), cannot hear that argument. It would only fit a company looking for a new fleet group now, and with everything not too far.