Aventura, a great place to research prices is
www.watchrecon.com. Some quick searches will give you an indication of prices.
For used watches, condition and service needs to be considered. Also, box and papers.
I've seen 111's go for 3200 to 5000+. They were all priced fairly as the condition, boxes, etc all need to be considered. For example, a G series should have been serviced by now. If Panerai does that you are looking at $400+. There is no definitive answer when dealing with used.
Good luck and enjoy the hunt. It really is half the fun!