Before setting the incentive structure, please keep in mind the SLAs you have agreed upon with the companies you are recruiting for. Ideally, performance should be measured against the company goals, and all efforts should contribute to the achievement of that goal. This will help drive productivity in the right direction. Keep goals simple to measure and ensure that everyone is informed of their targets and goals.
I assume the following: Recruiters are assigned positions that they need to fill, and there is no overlap of assignments. You have an agreed-upon cycle time and tenure with clients (i.e., an employee has to complete a certain tenure with the company before you get paid).
Target 1: Average Cycle time - 90% of people placed should be placed within "x" days of being given the profile. This will contribute to improving turnaround times and enhance your standing as a recruitment consultant.
Target 2: Fitment Rate - Number of candidates referred to Number of Candidates Hired. This will ensure that the right candidates are shortlisted and sent for interviews to clients, thus working towards building your image as a company that selects the right kind of people. Examine your current rate and set a goal of 10-20% higher.
Target 3: Confirmation Rate - Number of employees confirmed after probation (don't count people who were confirmed after an extended probation) to the number of people placed. Awards placing stable and performing candidates.
Target 4: Customer Satisfaction - Survey 5-10% of your clients for the candidates you have sent to them for an interview. The questionnaire is really simple to design; ask them if they are receiving the right kind of profiles and if they are satisfied with the recruitment services being provided by you. Also, have them suggest areas of improvement and what's going well. Additionally, call back placed employees and find out how satisfied they are with the jobs they have, including salary fitment, growth opportunities, relationship, and attitude of bosses.
These targets will not only help you work towards improving your standing in the industry but also encourage your recruiters to put in extra effort in shortlisting the right people rather than following a general mass-mail strategy.
Before assigning weightage to each, consider your major pain areas and distribute the weightage accordingly. These are my thoughts on what can help. You have the right to choose what you implement from these suggestions. I know all these are data-driven, but I believe that's the best way to measure consistently and accurately.