For agencies that offer just one product or when the objective is to measure performance on a product or program basis and when terms or numbers are not negotiable, the number of customers that an agent gets to sign up on an average is a measure of his or her performance.
For situations where the agent has the flexibility to negotiate numbers, the revenues resulting from his action could be a measure of his performance on that situation. Performance can then be obtained as an average over a week, month etc. While the number of customers acquired could be easily measured, the revenues that are directly attributable to an agent's action could be tricky to find.
The cost factors could be introduced if appropriate and thus profits could be used as performance measures. For example, some agents may have acquired more customers or collections using fewer calls or company resources and thus in a more cost efficient manner than others. These agents are obviously better than people who acquire the same number of customers or collect the same amount of money but using more resources.
A weighting (or discounted cash flow) approach could be employed in installment-based arrangements. A debt collector who sets up an arrangement of $20 monthly for a period of 6 months would be better than someone who arranges for $10 monthly installments for a period of 1 year. Same amount of money collected quicker is better.
But the idea is that depending on the industry or the purpose, different metrics could be used as measures of employee performance.