A lot depends on who you're buying it from. If you're buying it from a quality designer or platinumsmith who doesn't mess around, and provides the highest quality manufacture and workmanship, four thousand dollars is fine.
If you're after saving a few bucks, and going with a generic manufacturer of good quality, you might be able to find it for $3500.
Of course, every jewelers claim that they have the highest quality of workmanship. But, I just want to know what should be a fair price to pay for an eternity diamond band - F VS1, 2.75cts, platinum, channel set? Is paying $3500 too good? should I be skeptical if paying a lower price than that? Or, is $4000 too much or a reasonable price to pay?