Michael Scherer compares each candidates' spending to their vote totals:
So how much has the entire Romney campaign spent per vote received? $17.14, which is a lot more than the $2.54 that Santorum spent, or the $9.05 that Gingrich has spent, and only topped by the $31.55 that Paul spent. What do these numbers tell us? Both a lot and a little. We can say convincingly that Santorum has been overperforming, compared to the field, while Romney has been underperforming.