Opportunity cost was very little for me and all the other CS people I knew in undergraduate. The alternative for most people was doing tech support and making 24K/year with no growth potential (using local rates). Since the local rate for a BSCS is about 50K, there's no meaningful reason for the bulk of students to drop out. You'd have to have 20% year over year growth in income (not counting the bennies coming in with a salary job) to equal it.

In other words, for a computer science student - if you have a business raking in cash and you need to drop out to keep the money stream, you're losing out. Otherwise, stay in college.