I've use a process of prioritisation in projects for managing requrements before, based on a weighted system where we looked at the relative benefit of having a feature against not having a feature for the customer, and the comparing against the cost and complexity of implementation. It works to a point.
Alternativly, from a PRINCE2 perspective, I've used the project board and an escalated issue management procedure to get a steer for what should be prioritised. Its why they are there afterall....