Hardware selection consultant

Well, I've been asked to negotiate in behalf of my client with an hardware vendor for a large provision sold as part of a bigger project to a third party. What my client want is: completely satisfy the third party while saving as much as possible from the price accepted by the third party.
In situations like this I usually ask for a fixed amount depending on the estimated time to evaluate the requirements and negotiate with the vendor plus a percentage of the saved amount.
I would like to hear from you, how do you behave and if you apply a percentage how much it is.
Thank you.