For Presidential elections in the US the Electoral College makes the total number of votes for a candidate not really relevant. What matters is number of votes

For Presidential elections in the US the Electoral College makes the total number of votes for a candidate not really relevant. What matters is number of votes in a few states, and in some of those states it is only a votes in a few specific districts. Most of the spending was concentrated in those states or districts so if spending per vote numbers are meaningful it is probably only so in those states or districts. Voters in the states that don't matter know their vote for President doesn't matter, and so if there are no close races for Congress or for state and local offices they care about, and no ballot issues they have an opinion on many will stay home, further reducing the utility of nationwide vote totals as an indicator of anything.