The balance of white/black box testing is a decision you will ultimately make based on your budget, risk concerns, and internal policies. PEN Consultants can help you determine the balance of testing you need during the no-obligation scoping phase.
Bottom line: White box testing is always going to give you the best ROI. A security tester's objective is to help you find your weaknesses and address them.
For more information on this topic, read here: Gray Box vs. Black Box vs. White Box Testing