I'm in the process of getting an musical instrument insurance for my entire studio, including my modular systems. The marketplace here can give a good idea of a module's "street value". This is also handy if you want to check how much to pay for a module on another site like ebay/reverb/craigslist etc.
But if a module is not on offer at that time, you're out of luck, nothing to compare. Just wondering if there are more members out there who would be in favor of some kind of price guide/price history. Like selecting a time period and getting an csv file with all offered modules in that period that you can than sort by name/price/date etc. Or old listings get filed in a seperate "history section"
On the Dutch synth forum there's discussion topic where people ask eachother advice on how much to pay for something, not really scientific but hey, at least some support/guidance.