You probably dont need to go to deep with Google Analytics, as far as this, as you need sign ups, and this is what you need to measure. Sign up page, trial signup, one month signup, one year signup, probably... Stick to good countries only, and dont buy traffic from the other ones (at least at the beginning, but it is a good general rule of thumb): USA > Canada > EU > Other Europe are suppose to be the best (in this order). You look at other factors too like bouce rate, number of pages visited, time on site, but you pay for sign ups, and this is what you want to focus on (what domain name generates sign ups, and keep buying from them, but not indefinitely too, check more sites all over again after one month or so, as results will not always be the same).
Adult Media Buys ? THE ELITE ADULT MARKETING AUTHORITY! - I am not sure if this is good, it is $99 per month. Learn, read, and talk to as many people as you possibly can before spending first money. I've also heard about people who spend over $5k, before figuring out, more or less, what is what. You dont want to be that type of person, and it is not needed too I think. There is sources on the Net, and most importantly people from ad companies know exactly what is what, and they will tell you (especially / if you spend your money with them).
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Google Analytics, again, is very good for this, and something that you need to know. You can for example track people who come from purchased traffic / domains, and come back to your site later on, and sign up (all this can be tracked). There is a lot of other things to it too, and Google Analytics has been recently changed / improved. This is the most important thing for this. You know Google Analytics (very good) -> you have advantage, and you are on a good way to making profits. It all works in a certain way, and for certain reasons. If you understand it, and do it right, you should be making money (just like regular ads on the street, or in tv, they pay for it - people then buy their products - and the company profits - they dont pay for this ads, and lose money, at least they should not and it is not how it should work; it should work exactly the same way with this, if done right).
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IE. You buy traffic from domain-one.com, and Google Analytics sets cookies on these devices / PCs. You can then track what these people do, every time they visit your site, a month and three months from this time. So again, if you know how to use this information (and there is multiple different ways), you have the advantage. There is a lot of people / companies who do Google Analytics (Partners) consultation too.
You can probably negotiate prices with the brokers, if you buy in large volumes, and so on.
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Adult Media Buying
A lot of good books about Internet advertising on Amazon too. Again, this is a separate specialization, and it takes time to learn, practice, and do it right.