It looks like those numbers are hard to quantify, since South Korea's expenditures include e.g. salaries for Korean workers at US bases and the provision of non-cash resources and services:

http://www.latimes.com/nation/la-na-trump-allies-20160930-sn...

I suppose if the government in Seoul no longer felt it was worth the cost, they could stop paying and ask the Americans to leave.

EDIT: Oh yeah, why did you put "protection" in quotes? That's exactly what it is. The US doesn't pay North Korea to break the South's windows, if you'll excuse the mixed metaphor.