If you go to
ebay you can search on US FDCs then ignore all of the current auctions as most are fishing for buyers with high starting bids and filter for "SOLD" auctions. This will show you the actual prices paid. Filter again for "lots" where you will see large groups of FDCs and what they sold for. Individual FDCs of this time period can sell in the 50 cents to $2 range for most of them, but to get that you would need to put a great deal of work and patience into selling them in that way. The larger the lot, the lower the prices as there are enormous numbers of these available with a limited number of buyers. When you buy large lots or accumulations on
ebay you get them for their wholesale cost. For example, here is an auction that actually sold - the buyer paid $103.50 for 1,200 FDCs from the 1960s with Artcraft cachets (Artcraft is one of the better known and nicer cachet makers). That works out to ab out to about 8 and a half cents per FDC.
http://www.ebay.com/itm/Lot-of-1-20...AOSw3WxZvBkp As has already been mentioned, there are some less common cachet makers that can have a greater value. Also, FDCs that are autographed by the artist who designed the stamp or other VIPs who are related to the design can have increased values. No cachets, the slightest degree of damage or discoloration, addresses on any but the early FDCs, unrelated markings, etc. can reduce the values.
The potential problem you will likely run into is that some sellers who are not familiar with such transactions on
ebay may have an idea that their collection was an investment that has gone up in value from what they originally paid for them.