Wouldn't an essentially untargeted ad be the most expensive to run? Meaning that something like that will always be the top-spent?

Not necessarily. If you bid low, you're effectively picking up "remnant" inventory that no one else was willing to pay to target.

How “expensive” (in the sense of total spend) an ad is is entirely driven by what budget you allow for the order.

But without targeting constraints you will get essentially remnants and are unlikely to reach any of your actual target, e.g. undecided likely voters in swing states

A lot of nationwide ads aren't intended to directly influence voting. Rather they are campaign donation solicitations to get more money to run future ads targeted to undecided voters in swing states. Just about everyone 18+ nationwide could afford to make a small campaign contribution if they care about the outcome.