> but at the pleasure of the (in turn politically-appointed) secretary

Parent is correct. The amount varies from administration to administration. But if you really want to be an ambassador, you're well positioned if you bundle a few hundred thousand to a few million dollars for the winning campaign. (There are traditionally limits. You can't usually buy your way into the U.S. Embassy in Baghdad or London. But for the postings with limited security implications, where the focus is on trade, you're mostly hosting expensive parties for your post.)