I'd like to take the a different tack - that not only will it be expensive, the degree will be worthless. Talking about "free college" reflects the special economic idiocy of Bernie Sanders and the other idiots pushing the program.
A college degree is valuable - to the extent that it still is valuable - because not everyone has one. Not everyone can master the work. If everyone has an Associates or Bachelor's degree (depending on who's making the speech), it will become equivalent to today's High School diploma. You actually see this already happening in that the only degrees that help graduates in getting hired are the ones with strong skills imparted. There simply aren't many jobs for holders of degrees in Offended Minority Studies.
If college becomes "14th grade", the perks and higher pay will go to the Master's Degree holders, the Ph.D.s or some degree that doesn't exist yet. Or, as the case in an article Borepatch linked to, in South Korea where college is already free
A 2013 McKinsey study found that lifetime earnings for graduates of Korean private colleges were less than for workers with just a high-school diploma. The unemployment rate for new graduates has topped 30 percent.I'm fond of saying that the only "law" you'll find in a non-science classroom that almost rises to the rigidity of physical law is the law of supply and demand. In Korea, you see that when the supply of college graduates (weighted, I'm sure, by the fact that only a couple of percent of them can be in software or other engineering) is far more than the market demand, the price goes down.
There's also the matter of students gaming the system. After all, if they're being required to take "filler" classes that are really intended to spread the tuition money around, not "make them more well-rounded" as it's always claimed, they're going to do whatever they can to beat the system. Like hire an unemployed college professor to write their meaningless term papers for them. Students have been gaming the system for as long as there have been students. Likewise, companies like Unemployed Professors (.com!) have been around for a long time. You could always buy a term paper; and it's not that different from buying Cliff's Notes and copying it manually at your typewriter while adding bits of your own verbiage.
So just what is that degree really worth? The degree that everyone has, that didn't impart any real skills, and was filled with busy work? We see in Korea.