A billion dollars? About $3 per US citizen for the single most powerful position in the world, with saturation media coverage of the campaign for one to two years. Doesn’t seem like a lot. Trump spent under $400m, Hillary under $600m[1]. Bloomberg spent more on his primary campaign than Trump did from start to finish[2]. Steyer spent ~$200m and didn’t break 4% in Iowa[3]. Sure looks like money likes a winner rather than buying elections.
It's not $3 per vote though the math is much more complicated. For starters not everyone is registered or votes on any given election (even a presidential). Next you have to factor in the primary and electoral college. Votes in Florida and other swing states are much more 'expensive'.
[1]https://www.theguardian.com/us-news/2016/dec/09/trump-and-cl...
[2] https://www.thedailybeast.com/wheres-mike-bloomberg-billiona...
[3] https://www.forbes.com/sites/michelatindera/2020/02/01/billi...