Stamp Price In Catalog

Please tell me if my thinking is correct.

As an example, the Issue is UN stamp set Scott #C1-C4. To apply the FDC price of $24.50, I need to have one of each denomination. If I only have one stamp of one denomination, but several stamps of other denominations, I can only apply the price for one stamp, and the additional stamps are in effect, valueless. And the same logic would apply for only one and multiple Used stamps.

Post some images of these covers so we can get a better idea of what you're asking.

However, take the catalog values of United Nations stamps and FDCs with a grain salt. The market is flooded with U.N. and demand is low.
You can buy at this moment a set of 4 U.N. FDCs, Scott C1-C4 for $3.95 and even then it's a tough sale.

https://www.ebay.com/itm/C1-C4-Unit...!12090!US!-1