Calculating ROI is one of 
the basic tenets of PPC, and yet many 
advertisers don’t consider it or even understand it. 
A lot of advertisers perform 
campaign optimizations based solely on 
conversion rate or cost per 
conversion, choosing the ads and keywords with the best metric and calling it a day.
For ecommerce goal analysis the method of calculation is similar. The  biggest difference in the Ecommerce data is that it’s real and accurate  compared to a lead form’s estimated goal value.
In Adwords, we can look at the money we spent to reach our goals in the  same time frame. Compare that with the purchases made through CPC and we  have a precise number we can confidently call ROI.
Lead GenerationConversion rates (found in 
Google Analytics)
Conversions (found in Google Analytics)
ECommerce
ROI (found in 
Google Analytics and Adwords)
Revenue (found in Google Analytics)
I’ve always found that ROI is one of those terms that has been over-used  and abused by so many people and as such, there is confusion on how  best to calculate it.  Personally, I like to use the following formula  when we are discussing the ROI for any PPC campaign:
ROI = [Contribution] / [Cost]
So to calculate Contribution for a PPC campaign:
([Your average profit per sale] x [Estimated number of Conversions]) – [PPC Spend]
To demonstrate more fully, let’s take the following example:
Monthly PPC Spend:                    £1,500
Average Profit per Sale:                 £50
Number of Conversions (Sales) per Month:  75
and so the Contribution to Margin of the PPC campaign is:
(£50 x 75) – £1500 = £2,250
and your ROI would be:
£2,250 / £1500 = 150%
Phew!  But there is an easier way.   We have just created an ROI  estimator / calculator spreadsheet that you can now download for free.   We hope that it will be a useful tool for you when reviewing your PPC  campaigns.