When going for a CS degree, the usual rule of thumb is one should spend three hours studying for every hour in the classroom.
To get the education you'll get at a college, you'll have to spend those three years studying any how. So college is really like three years of independent study and one year of class time.
Why not just spend that year of time going to classes, learning things from professors (some of whom are quite knowledgeable, and doing cutting edge CS theory research), learning things from classmates and making social and networking connections? Perhaps private schools are expensive, but UC Berkeley, UIUC, UWashington, Georgia Tech are more affordable, especially with Pell grants etc. Especially part-time.
Also - people doing self-study tend to go right for the immediately useful stuff - how to make web pages in PHP and the like. How many people spend eight months studying calculus, then four months studying discrete math, then four months on graph theory, then four months on theory of computation, then four months studying logic gates and processors, then four months studying data structures etc.? What kind of code is someone who has not studied concurrent programming going to write when an application needs threading?
A degree is also a sign someone can stick with something for four years.
Of course a degree is no guarantee they know anything in and of itself.