To figure that out on my own, I use eBay and the advertising parts dealers to figure out a "fair market" value on stuff. On eBay, the value fluctuates seasonably it seems. Spring and Summer bring higher prices than fall and winter. The parts dealers have steady prices but I don't consider them to be "fair market" as they are more "supply and demand" driven. Lately, I rely more on eBay for market value prices and the dealers for ceiling prices. I guess it depends on what method you are going to use to sell them. If using eBay, research using the "completed auctions" to get a feel for what is selling, in what condition, and at what price. I wouldn't go by the dealer's prices unless you were prepared to back it up like the dealers do.
You said the carb is NOS. Some NOS parts with gaskets need rebuilt depending on how they were stored. I got one with governor a couple of years back (eBay) that was used for $30 that only needed a rebuild. It is on my truck and running perfect today. I agree with Longhunter 7, a NOS one, not dried out or needing rebuilt, may fetch about $200. Does it have the governor on it?