This is not possible. And I agree that you thinking that it can be done probably shows that you are likely to be going to provide services without sufficient experience...
per-keyword, per-position basis
What are you going to do anyway? I for one used to charge per service and hours I spent doing some particular task, but I was never good at that (it always felt like I was working for too many hours without too much work done) so I guess I ended up charging too little
With all due respect, I'd disagree. Again, I am not that good at providing services and stopped doing SEO for clients long ago but there's so much more to starting making money than SEO that you can't really control that...
Further you may not know it but your job is not to rank a keyword for the website, your job is to MAKE MONEY for the website through increased internet visibility.
Everything will be ok in the end
If it's not ok, it's not the end