Rob
Most of what i have collected are the older ignition and diesel engines mostly before 1948. best way to find a value is on Ebay Lots of them there. Usualy some one has to die for some to become available. There are a lot of historical and low production engines not for sale otherwise. Quality holds its value like guns but rarity also drives the prices up. I've been building models since in grade school 1940s. Last count 485 complete runable engines.
Tom