How to track how many opt-in leads I bring a client?

I want to charge a certain rate per lead that I bring in clients. Are there any ways to track how many opt-in leads I bring in for them?

There are many ways to track anything you want. It all depends on exactly how
you are bringing the leads in. If you're doing print, or offline, promotions, it could
be as simple as setting up a special page to send them to. If you're using online
promotion, a simple tracking code could suffice.

Since I do not know your exact situation, I will elaborate on both.

OFFLINE:
Let's say that you're creating newspaper ads, or flyer ads. Rather than linking
directly to the website home page, you should link to an internal page of the
website so you can check the stats to see exactly what amount of traffic is
entering the site through that page. It would benefit you greatly to set this
page up as a lead capture page -- have some sort of opt-in form and free
offer to present. The URL to the page should be named oddly enough that
not just anyone will visit the page, but normal enough so that it does not
deter the user from typing in the URL. If finances allow, you could even
register different variations of the domain and use simple redirects once
the user has opted in using the opt in form.

ONLINE:
If you're referring people online, a simple tracking code within the link
should be enough to track them. Something as simple as a link such as
http://domain.com/?ref=referral-code would work just fine. You can then
view the sites stats (most hosting providers provide free statistic software
that will enable you to view the stats of everyone visiting the site). I would
still attempt to link to some sort of opt-in page, which is only for people who
have your link. This will give you a better idea of how well leads are actually
converting as it will tell you which leads are action takers and which ones are
simply window shoppers.

Because you did not provide too much information, the information I've just provided
could be exactly what you were looking for, or, I could be way off base, LOL. In either
case, please take a moment to post back and let me know which is the case. If I'm
way off base, please present us with additional information and I'd be happy to reply
accordingly.