An excellent Memorial Day weekend Cre8asite Forums thread was started by a novice SEO that has the requirement of showing to her company, how SEO can impact their bottom line.
She express that she is "missing something somewhere" because she is unable to come up with a solid number or percentage. Meaning, she wants to say, if they invest $X dollars in SEO or $X in PPC, it will return an increase sales of X percent.
While most of the replies in the forums are supportive, saying that it is almost impossible to come up with that number, without giving it a try first. Why? As one Cre8asite Moderator explains, "because the numbers are hugely variable for every site, and you really don't know the effect until after you've made the changes."
But I believe, as do others, that you can give ranges. Cre8asite moderator, Barry Welford, a statistics lover (I love stats also, but probably not as much as him) says it is possible. I agree.
Basically, look at the numbers you have now. If you have X visits per day and X of those current visits turn into sales or leads - then just multiple that number. In fact, you will probably see more targeted visitors with SEM, so if you like, you can even increase the current percentage of sales/leads driven by your current web site and use those figures. It is probably best to give a low, medium and high range. You can then use your current formula from lead to sales to dollar average, to come up with an ROI on a specific campaign.
Those will be your initial numbers. Run a test campaign. Then you may just be surprised by how your numbers align with your estimates. Your initial report is just to get you some money from the company to prove to them that it can work. Once you get that money, then it is up to you to make it happen.
Forum discussion at Cre8asite Forums.