moving in the range of 3.0-4.0M$ is ok, the lost of value in the next years is not as much if you were buying a new 60 (13.0M$) where you will loose half of that in less than 5 years.
Also buying a Falcon 20 for 1.0M$ with high cost per hour is not good if you plan to fly 600hrs/year, we are talking almost 1.0M$ extra in costs every year compare to a 60 or Hawker, but if you fly 100hrs probaly is your aircraft.
As always depends what you need and what you plan. When these rich guys come and want to buy an aircraft always think on a G550 and normally end buying a Citation 1. Calculate which is their fortune and maximum 5% of the total is what they should expend buying an aircraft