Total Cost to Exercise
The total cost to exercise is the actual cost you would have to pay if you wanted to exercise your options at a given point in time. The total cost includes the cumulative costs of fees, tax withholding, the exercise cost, and other potential fees associated with that option as defined by the relevant parties.
Andy Rachleff's When Should You Exercise Your Stock Options? at Wealthfront
Max Schireson's Startup stock options explained at Max Schireson's blog