Hi Ryan:
It is a murky topic, and even with new mics there are 7 levels of pricing: List, MAP, Discount, Wholesale, Employee, Cost and Blowout. I find it useful to simply ask, how much does a new one cost? The retail price gives you a general idea of the consumer cost of an instant replacement.
Used prices are even more volatile, because nobody is enforcing MAP -- and you can certainly buy a used pair of mics at a bargain price, sell one at the normal price, and keep the other one with $0 as your unit cost.
I agree that you are always buying more than sonics. Especially with vintage gear, there is an emotional component that is very strong, not to mention history, brand name, star power, advertising, and hype. This is usually what the market is selling, and why it is unreliable.
Stephen