Please forgive me if this point has been covered and I missed it while skimming through the responses above.
One critical point that needs to be looked at is the site itself.
No good getting users to the site if the site's design/layout is not optimized to generate sales/leads, etc.
I've worked with a few sites where the goal was to positively influence inbound traffic from search sources. basically, they wanted better rankings on Google. I usually explain that it's not better rankings per se that you might want, but trackable results such as 20% more inbound traffic from the engines overall.
So, I'm going to suggest you split this hunt into two segments:
1 - take a look at your traffic logs and determine what volume of traffic you currently get is from inbound search traffic. Most analytics packages should be able to show you these numbers.
2 - review the site with an eye towards improving the sales funnel. Are there barriers such as;
~ a prominent phone number shown instead of an obvious online path
~ hidden pathways - build a sign up section to look too much like the rest of the site and new users may miss it
~ asking for too much information up front - asking users for a lot of personal info up front is a turn off for many
~ number of steps in a process - make the process too long and you'll lose users along the way
Anyway, my point is that increasing inbound search traffic is only one variable in the conversion chain. So, tread carefully when trying to determine dollar values when pitching execs. SEO and it's results are but one tool in the Marketing department's tool box.
I personally stick to using my seo methods as a way to help with inbound traffic numbers.
If you can, ask the sales department what an average conversion is worth to your company. You can use THAT data to project "what if" scenarios. If the average conversion is worth $500, and you know the current site converts at a 3% rate, then any MORE traffic your SEO efforts generate will likely follow the pattern.
10,000 visits from search a month now = 300 conversion @ $500 = $150,000/month in revenue
Let's say your search optimization efforts alone net a 15% increase in inbound traffic:
11,500 visits from search a month then = 345 conversion @ $500 = $172,500/month in revenue
So, obviously it's worth it to your company, should the numbers be realistic. Couple this with reviewing the site and optimizing the sales funnel and you could see even greater returns.
Now, on the cost side, we have:
You.
Your salary, your benefits, the costs of having you in place (computer, phone, etc.)
...and in reality, we also must account for the other folks in your business you'll need time from - IT folks, SysAdmins, Product Managers, etc. (as applicable). Each hour of anyone's time increases the cost to your business of getting the project (search optimization) up and running.
In the end, experience has taught many of us that the return is easily well worth the investment.
If your company requires some concrete numbers before unleashing you with funding, two things will need to happen:
1 - you will need to give them some level of comfort that this is not a black hole for funds - which I think you can do.

2 - you will need to educate them that there is nothing you currently possess (as a company) that you can use to say, "If we spend this, we get this back." Running a well optimized website can be very different from running a poorly optimized website - in terms of traffic...and traffic = revenue.
At some point they will need to have faith in what you say. Getting an optimization campaign running from the inside is very much a sell-job at times. But in the end, YOU could legitimately come out a hero.