Awhile back, a complete unknown named Robert Stooksbury ran as a Dem against the longtime incumbent, Tom Price. He didn't spend a dime except the filing fee (and for all I know he may have filed a poverty affidavit) and had NO advertising at all. He got 119,536 votes doing absolutely nothing.
So Ossoff spent $22M to get 4,357 votes. By my calculations, that's $5,049.35 per vote (if you round up.)