I saved for a long time and then held out for a long time when I was shopping and found my 05 Cummins 6-speed with 99,000 miles for $24k. It was a little more than KBB said it was worth but I also knew what they really went for on the market so I was satisfied. Paid cash happily knowing that I had several hundred thousand towing miles left and no payments. That was about 4 years ago. Last tow rig I plan to own. By the time it is worn out they'll probably have electric tow rigs that have 20,000 ft/lbs or torque or something. I've had guys at gas stations offer me $25,000 for it twice now. Don't know how serious the offers were but it makes me happy about what I paid.
At the time I bought it I was driving a 1994 Accord as my DD with 350,000 miles on it. I drove it to Arkansas to get my truck, rented a u-haul, and towed it home with the truck that I bought.