Most of what is posted is listed in the Scott Specialized Catalog of US Stamps (United Nations Section) as being valued at $1. Generally, blocks are worth 1.5x that amount, or $1.50. Since most common material found in the Scott catalog is discounted by 50% to 70% on the secondary market, a value of $1.00 per item shown is generous. Most buyers won't even offer that. On the other hand, if you plan to sell on
ebay or another on-line marketplace, then $1 is probably a reasonable starting point.
Mass production and limited collector interest in UN stamps has made most UN stamps next to worthless, unless you're a specialized collector or unless some special handpainted cachets are involved. Too bad, really, as the UN has produced some nice looking stamps, but they never quite held much interest in the secondary collector's market.
[edit] I just checked
ebay and there are at least 90+ UN first day covers from 1963 for sale, most at a BIN price of $1. No bids on any of them, which demonstrates just how low the demand is for such items.