The 'cost' of voting isn't about paying money to vote. It's about the laws and policies that are brought in by the people in power. Taxes are a very obvious direct one, but just diverting tax money from "good stuff" to "bad stuff" is a fairly direct cost too.

Someone promising you unreasonable things is much more likely to be fucking you over. Some would say that promising you unreasonable things and then fucking you over is the basic thread of all politics, Trump is just taking it several steps further because apparently people have become unmoored from the reality of government and so cannot judge what is unrealistic anymore.