Your best bet is to use G's keyword planner tool... Although its primarily used for Adwords it will give you an idea of the value of traffic you get for each keyword had you paid for each click... In addition the keyword planner tool provides estimate of search amount per month... Make sure you use exact match data when examining search stats within the keyword tool and assume roughly 40% of that value for first position... Here is a made up example:
Keyword = "blue elephant apples"
Cost per click = $1
Searches per month = 1000, Noting 40%=600 assuming thats the traffic for first position - this gives a broader idea of the possibilities should you increase your rankings...
Lets say from your traffic stats you get 230 visitors whom landed on your website with searching the term "blue elephant apples" on the serps...
That equates to a value of $230 per month ($1 * 230) - had you paid for it using Adwords... As you can see you're probably not in first position ...
If you can try taking it further with seeing how many of those 230 visitors purchased your product or signed up to your list.. Lets say out of the 230, 50 people bought a $50 product... That equates to a profit of 50*50=$2500... with a conversion rate at 50/230 = 21%...
Now compare that to the total earned revenue...Lets say you earned $5000 in a month.. Out of that 50% is income earned from ranking for the term "blue elephant apples" ...
As you can see its up to you how far you want to take your analysis... Bottom line if you take it to this level then you need to be tracking everything! Hope that helps...