here you find the doc to the api one may get costs for googles adwords from
https://developers.google.com/adwords/api/docs/appendix/reports

of course this function would not only be interesting for ecommerce websites but for everyone who pays for (a part of) his visitors and have goals to reach on his website

so there should be an overview where you could see the costs to get visitors to your goals (ecommerce is one of them)

in general there are 4 main types of paid campaigns which should be covered

1. pay per click
   (every visitor costs a small amount of mon. (<= the complete word seems to be listed as spam here) e.g. 0.55USD)
2. pay once for all visitors within a period
   (e.g. 100USD per month, if your site receives 1000 visitors each cost you 0.10USD)
3. pay once for all visitors
   (e.g. you spend 300USD for a company to make and send your email newsletter, if your site receives 100 visitors each cost you 3.00USD)
4. affiliate programs: pay a part of your revenue -and only if there is a revenue
   (e.g. you got a visitor from this campaign to your site who makes 50,00USD revenue (ecommerce basket) and you have to pay 10% from this to the affiliate who send this visitor to you that would be 5,00USD for this visitor

=> to handle this on should have the possibility to specify the type of each campaign and based on this selection you need a possibility to put in
->1) -> a source to get the CPC for each visitor from
->2) -> period selector: day/week/month/year and a field for the price for this period
->3) -> only a field for the price
->4) -> a field to put in the affiliate rate e.g. 10%

Thanks for the suggestion. This would indeed be a very useful feature!

to think this topic even further / more abstract:

..in a second step
one could not only use this to analyse how much a visitor from an external source costs (from paid sources), but one could define also costs or in other words effort in form of working hours.

Doing this one could give e.g. a number of working hours for writing and publishing a blog post on your site.
Logging the number of visitors which come from external to this site you could get something like hours of work spend for each visitor or if theses visitors visite other sites of your website and reach goals you could also have for these visitors something like revenue per working hour

having this feature within piwik would make piwik unique and outstanding
-because I could not imagine that others like GA will follow and make the cost-efficiency of online-ads that much transparent.

On the other hand I think this is a feature which would be very interesting especially for everyone running a commercial site.
And these people have and may spent some mon ey for crowdfunding this feature...if we give them a possibility to do so.

"Customer Acquisition Cost (CAC)"

=> Since this is really important and would make piwik unique and even more interesting for commercial websites: maybe one should set up a campaign and look for sponsors for this feature (like it was done in the past for other great features..)
the set "long term milestone" is simply too far in the future!!