Quoted from Brijam:
It would be nice if the algorithm was made public, though. I'd love to see an interactive website like KBB somewhere with fields for certain mods that are tied to the sale price, so you can really see what a certain mod does to the resale price of a pin. You know, similar to how most home improvements don't add the full value of their cost to the house. So for example, I see people routinely discount the full $400 for a ColorDMD if you want it removed and replaced with the old DMD. On the other hand I personally subtract about $300 if I see a machine with an LED "Skittles treatment" since I'll have to replace most if not all of the LEDs since I only use white for GI. What does having a CPR playfield do to the price on average?
Then you could theoretically plug in your machine and the mods you had and get a fair estimate of value.
I like the concept, but I doubt there are enough pinball machines being sold to make that analysis (I was admittedly a history major and have very poor math skills, so I'd be happy to be proven wrong on this!).
Any machine that sells will have variables like other mods installed and condition of the playfield, boards, and cab, that make it hard to pick out what affect an individual mod has on the sale price. If enough of each title were selling each year you could use statistical analysis to make a meaningful assessment about the market value of individual mods. As is done with existing homes, which have the same problem of many variables, but have the benefit of millions of sales per year providing lots of data to work with. But given the tiny size of the pinball marketplace, I think we're mostly stuck with gut-feeling assessments based on anecdotal evidence.
ColorDMD seems like a unique case, since you can move it game to game and it seems to be universally respected. So even with limited data I'd feel pretty safe saying ColorDMD keeps 80%+ of its value on resale. Much harder to say with other mods.