An "approximate" date can be deduced, based on the button's various charactreistics:
1- the button is a 1-piece "flat button"
2- the button's backmark is "raised" letters
3- the emblem portrays an eagle with "skinny" raised wings, and the motto "E Pluribus Unum"
That particular version of US Diplomatic Corps button and emblem seems to date from the very-early 1800s ...approximately 1805 to perhaps as late as 1820. Those buttons were manufactured in England for purchase by the US government because the infant American button-making industry was not yet capable of mass-producing that very-detailed emblem on a brass button.
Value always depends on three factors: rarity, condition, and "demand" (meaning, level of collector interest). The OD-5 buttons are rare, but in excavated condition do not have as much value as non-excavated specimens. In America, the most-highly-valued buttons are Civil War era Military buttons, because the "demand" for them is high. Far fewer people collect early-1800s Military buttons ...and even fewer people are interested in buying Diplomat uniform buttons. That is why Diplomat buttons are not listed in the North/South Trader's Price Guide. Your best option is to do a websearch to find examples of your OD-5 button that are for sale, or have been sold by an Auction house.