If you're considering hiring a marketing partner, there are really only two options for accountability. And neither necessitates "paying for performance."
"Measuring by their results" basically means that you judge your vendor's performance based on their ability to successfully deliver their product or service. For example:
"Measuring by their results" assumes that the vendor can only control what they do, not whether or not it results in more sales.
"Measuring by your results" means that you judge your vendor's performance based on solution's impact on your sales and profitability. For example:
"Measuring by your results" assumes that the vendor has some involvement in and control over the broader marketing and sales processes into which their solutions fits. And it assumes that that involvement or control increases the probability that their solution will have a positive impact on your sales.
One irony in all this, of course, is that many companies hold their vendors accountable for sales impact without actually involving them in the broader marketing and sales processes that would enable the programs to have a positive impact. If it's "fee paid," the vendor generally doesn't care. But too often companies simply try to offset their risk without giving up the control.
But the greater tragedy occurs when the company insists on "paying for performance," but without giving the vendor the resources and tools to do the job. In our experience, failure under a "pay for performance" is rarely a matter of inadequate incentives. Rather, it is almost always a management problem.