Before submitting a proposal, you need to know a few details. For example;
What is the number of positions and time frame that they need them to be filled?
Is price the key factor or is quality or something else their key factory in making a decision?
Have they had experience with other services? If so, what was the most frustrating part of the service they received and what was the most impressive part?
Once you get a handle on these types of issues you can craft a proposal to address price, terms etc.