My wife got me a very good condition, clean 1964 Group 3 Root Briar for $160 (might have been $165ish). So that should give you a fair idea of a ballpark estimate as to what you should pay. Look to spend $150 to $300, but I would personally keep it around the $200 range or less unless the pipe pipe is mint or excellent. Places like Smoking Pipes, as much as I love them, oftentimes price their estate Dunhill pipes very high, so I wouldn’t use that as a reference.
The Dunhill Master of this forum, Foggymountain (who recently passed away), had hundreds of Dunhills, and he said that the best era for Dunhill was the 21st century. So essentially, the new Dunhill pipes. I have a small collection of Dunhill pipes, and from my limited experience I would agree with him. My 2014 Dunhill Shell is the best smoker. Now, when my Dunhill Patent comes in I may change my mind, but so far the 2014 Dunhill is superior to my 1960s Dunhills.
This is my novice Dunhill collector’s advice, and others may disagree with me.