16 hours of driving, let's say averaging 70mph, is 1120 miles. Even at a moderately shitty mpg for a modern car of 30mpg, that's 37.3gal. Around most of the Midwest I can count on gas averaging $3.50 or less a gallon, so that's $130 each way. That's cheaper than anything other than extreme budget airlines, if you don't need any luggage whatsoever. If you have more than a carry on, then you're already saving money.

I'm basing this off Midwestern multi state drives, since that's what I've done the most. I can get a cheaper flight on Frontier or Spirit, but again, that falls apart as soon as I add any luggage, and I prefer to travel with luggage when it's a reasonable option.