There are many different types of spokesperson deals out there and each celebrity is different in how they approach these deals. Here are some of the main ones:
*Voiceover only (for a flat fee)
*Commercial or Infomercial Video and Appearance (for a Flat Fee or Fee plus percentage of sales or equity )
*Full Equity Partner in the brand/project
*Using or not using their name and likeness in future promotions, online and offline
Forget about paying on a CPA basis, there is always some cost up front. No real celebrity is going to work for free and hope that something comes out of it, they usually have plenty of other better opportunities on the table. This is not to say that you can't have an incentive for the celebrity to go above and beyond and get rewarded for it, the George Forman Grill is a great case study in this regard.
Some celebrities are a bit inflexible but their agent usually knows the basic framework that they usually deal in, which is at least a starting point for negotiations. If you do not have the money lined up please do not start this process, because if you do you will ruin your future chances with that celebrity and many others as the industry grapevine is very real. If you do sign a contract, you will have to usually wire funds within a certain time before everything starts.
Celebrities can add a great deal to your business, but just make sure that the deal makes sense to you from a business perspective, and that the numbers won't put an undue strain on the business. If endorsements didn't ROI companies wouldn't continue to do them, but that does not mean every endorsement is successful out of the gate.
Business Events set up and advice
Social Media promotions
Legal & Contracts
More legal and contracts