"Performance-based" marketing agencies profess to "not get paid unless they get you results." Be careful with this approach, here's why.
I once had a prospective customer say to me, "if you can't guarantee me leads, then I can't guarantee you my money." I was a little bit taken aback by that comment. Clearly, this person probably hadn't done much marketing or advertising before with a professional where it turned out well.
If you're going to make claims or work on a performance basis, it better not be payment for leads. It needs to be payment for closed customers. We're willing to do that in very limited cases where the systems are in place for our team to see what a customer actually spent in near-real-time.
If that system isn't there I'd reply, "I can't guarantee you any customers if you can't guarantee that I'll have a way to hold YOU totally accountable as well." Additionally, I'd need to know that your operations and delivery was top notch and capable of keeping up with that sales and marketing machine I'd like to build - otherwise, it isn't worth our time.
Are you willing to let an agency see every detail of your customer relationship? Some companies are willing to do that, but if you're not you may want to stick to the old method of finding the right person for the job and paying them for their time just like most common transactions where a business hires a professional service provider.
But for the sake of all that is holy, don't pay for leads. Please, just don't do it. Ever! If you're going to go performance-based pay for closed deals and make sure it's with a person or agency you really really trust because at that point, you're darn near partners and if the agency is smart, they're going to look for a long-term contract that is difficult to terminate.