I'll use an example:
When I want to track sales, I create a link that looks something like this (note the end):www.domain.com/index.php?src=mothering
Then when someone clicks on that link, my website keeps track of the "src" part which in this case equals "mothering" If they go on to my order page, the "src" is carried with them. When I get an order, part of what I get is the "src" value, in this case, Mothering.
So then I know that sale came from somoene who clicked on a link I had at Mothering.com. I keep track of every single sale and where it came from. In this way, I can tell which of my ads are no longer effective and which make me the most money.
I can tell you, that I was extremely surprised by my results. Ads that got high click through rates did not always equal greater sales. Ads that got lower clicks resulted in better sales.
Now I know if paying for a sig ad is worth it or not because I've been trackign the effectiveness of these ads.
Okay, thing about the php code is that you have to set up the php code on your sales pages. This is tricky if you don't know php. I can do it for anyone who wants me to.
PM me if you're interested. It's not cheap though... however, I have found it to be invaluable because I've stopped wasting my money paying for ads that turned out to be ineffective.
So you can either spend the money on ads that you don't know are effective, or you can set up this tracking system and for years to come you will only spend money on ads that work. So in that sense, the set up fee is way worth it.