I won't quote you a dollar value, but if someone interested in the gun is a retired Secret Service Agent it might be worth a hundred or two more than the same gun with no markings. To most non SS Agents, probably the same amount less than the same unmarked gun.
I have found over the years that marked guns appeal to very few people who were not affiliated or associated in some way with said agency and tend to actually be worth less than a plain Jane version.
Chief38