There are so many variables that will screw up any rule of thumb.
Geography, scarcity, type of machine, initial quality, and whether you can still make money with one, just to start.
For example, a new Monarch 10EE would be somewhere between $50k and $60k. There is a late model (80's) well tooled, supposedly almost new one for sale in LA right now at a dealer for $29k. Well, by your rule of thumb, thats about 50%- but it still seems like a lot to me.
Some tools are incredible deals, because they are oddball, or obsolete, or just not popular. Moore Jig Bores that would cost $75k to $100k new, selling for $2500.
Other tools like Haas VMC's have an unreasonably high resale value- sometimes as high as 95% of new.
With tools that are ten years old or newer, its the rare dealer that will go as low as 50%. They shoot for more, often lots more.