Affordable colleges, in theory, mean that more people are graduating with a degree. This doesn't reduce the value of the degree, but if everyone graduated with say, a Psychology degree and the job market was flooded with psychologists, the argument could be made that the degree is near worthless (you have little to no chance of actually getting a job with the degree).
In reality, however, there are so many degrees, and each degree leads to so many different career choices, unless everyone benefiting from affordable colleges (those who would not have attended if it was normal price) flocked to only a handful of specific degrees, the "value" of degrees would not diminish. It just may become more expected that, in a job application, you have a degree.
That's my take on it.