This calculator, inspired by the April 7, 2014 episode of EconTalk, takes into account the fact that not everyone who starts college earns a degree. You'll need to estimate how likely you are to earn a degree, the salary you could earn without one, and the salary you would expect to earn with a degree.
How likely you are to graduate depends on you, your major, and the college you choose. According to the National Center for Education Statistics, 40-45% of students who begin college don't finish. The more selective the college, the more likely its students are to graduate, (which might say more about the type of students these institutions admit than the institutions themselves.) Here is a degree completion rate calculator from UCLA that takes into account your GPA, test scores, and ethnicity. This calculator is based on UCLA's matriculation rate data. Here are a few example data sources for exploring how graduation rate varies by college and by major:
Here is a source to inform your estimate for starting salary as a college graduate. Payscale.com's College Salary Report 2014.
Thanks to Ryerson University for the example form from which I adapted this.