economics question

As vice president of sales for a rapidly growing company, you are grappling with the question of expanding the size of your direct sales force (from its current level of 60  national salespeople). You are considering hiring from 5 to 10 additional personnel. 

How would you estimate the additional dollar cost of each additional salesperson? Based on your company’s past sales experience, how would you estimate the expected net revenue generated by an additional salesperson? (Be specific about the information you might use to derive this estimate.) How would you use these cost and revenue estimates to determine whether a sales force increase (or possibly a decrease) is warranted?