For reference, working 40 years at $30,000 a year will earn you $1.2 million. Earning $640,000, or ~53%, more is certainly substantial.<p>Edit: The $30,000&#x2F;year reflects current incomes, the $640,000 lifetime difference is based on 1987 dollars, like @twoodfin pointed out.  
It works out to greater than double lifetime earnings using the adjusted $1.3 million figure.