What you need to do is work out your per visitor revenue ($ index).
Once you know how much you make per visitor you can assess whether the incremental revenue from an Adwords campaign is worthwhile or the opportunity cost is too high.
ie. you know you make $.72 from each person that visits your site, Adwords data shows that you will be paying $.42/click on average. Therefore you can stand to make $.3 per additional click generated via Adwords.
On average No.1 Organic receives 42% of overall traffic and No.1 Adwords receives aprox 8%. Therefore no1 in both will lead you you capturing 50% of the total traffic for a given keyword.
This is where it becomes tricky...
You need to weigh up the opportunity cost of purchasing that Adwords traffic - to do this you take into account your account type with Google (preferably 30day on CC) and how you are generating profit from the site. ie (Adwords net 30 etc).
Now you work out the time between paying Google for the clicks and the time it will take to get your funds. This could be anywhere between 7 and 60 days.
Now, if we secure 100 clicks per day at $.42 we are spending $42/day or $1260/month. If, like in most cases, you can pay Google at the end of the month, and then have to wait for a Net 30 payout, you will have an overall debt of $1260 for 30 days.
However after the 30 days, we will be receiving ~$2160. Meaning that we have an ROI of 70% delayed by 30 days.
The question you need to ask yourself is..... If I had $1260 in my bank account, what else could I do with it, and consequently how much money could I make as a result.
You need to assess if the alternate option will
A) Make more or less money
B) The period of time you are out of pocket
c) The risk profile of any alternate uses for the $1260
In about 80% of cases the PPC number are more solid than any other options, and you can be confident in the numbers because you already have a profitable site in the niche.
Hope this helps a little.
Cheers
-aReJay