GIIB and G3 great airplanes in its day. The world has changed unless you want to stay in Africa and even then with limitations.
And for $400.000 to 600.000 a year you would be hard pressed to own a LR 25 and fly it for 250 hours a year like you say. Then multiply fuel, insurance, parking/handling(outside US), maintenance costs by 2 x 2.5. Pilot salary would be 33% more. Etc.
In a nut shell, Forest Gump would probably have a great one liner to why they are so cheap. Maybe something like " there aint no buck for half a buck".
Find out essentials like RVSM cost and feasability, specially in GII GIIB( cant go anywhere on straight jets at FL 260/270), Hush Kit cost, and Hot section/overhaul costs on Speys. Then worry about future tightening on restrictions, and resale value of A/C ( even with all above work).
Thats why it's cheap. It isn't really.
Why is a 10 year old Maserati so cheap?