It should not be hard to put together actual numbers on link building.
I use Google Webmaster Tools.
So far it is free & shows me the most information.
I ran it last week & is showed 76 domains with a total of 2000 links.
This week 76 domains & 2098 links.
Do that with what ever software you are using.
ALSO include the links you have applied for.
You don't have to write an essay use a chart with numbers.
6 Guest blog posts.
5 links from community directories.
100 links from fiver.
If he wants to know exactly how many links will gain him 1000 more unique hits in traffic, again that is a conversation that should have been had before you started with this client.
Client expectations are the job of the vendor.
If he is a retail client then he wants his phone to be rigging more often.
Your job is to get the site to a place where it is.