Walker's gone, and Bush ended up in sixth place Monday night, pulling in less than 3 percent of the vote. Between April and the caucus, Bush's campaign spent $27.4 million dollars (according to FEC reports) and dropped 11 points in the polls. Not all of that spending was in Iowa, but Bush refused to give up on the state — and it cost him.
Donald Trump ran a much more cost-effective race in Iowa, leveraging the fact that he generates free media attention the way Iowa generates ethanol. If we compare how much each candidate spent through the end of 2015 (the most recent data that's available) with the number of votes each received, Trump and Bush are at opposite ends of the spectrum.
For context: Rick Perry ended up spending about a grand per vote in 2012.
We must point out, though, that Bush didn't actually fare the worst. That privilege goes to former Virginia governor Jim Gilmore, who came in last in Iowa, trailing even "other." Gilmore spent more than $100,000 to garner 12 votes -- though as he noted during his surprisingly buzzy appearance in the last debate, Gilmore is focusing on New Hampshire.
There's one more thing missing from this picture: spending by super PACs on behalf of the candidates. Bush's super PAC has spent far more than he has. By that standard, his campaign looks positively cost-effective.