The question is not what is the ROI for Inbound Internet Marketing but how does the ROI compare to traditional marketing.
When a client asked what is the ROI of Inbound Internet Marketing I always answer the question in context to the ROI of traditional marketing (radio, TV, newspapers and out of home).
I have worked in buying and selling traditional marketing for a long time. Tracking ROI for traditional marketing is not easy. Let me give you an example from my selling radio days.
I worked for the Country station in Boston (WKLB). I had an event at a car dealer. I was the only media outlet that was advertising the event. I went around the crowd (just out of curiosity) and asked where they heard about this event.
Number one answer was WBZ (radio). Second most popular answer was the Boston Globe. In fact WKLB was the fourth or fifth top answer!
Customers deal with top of mind awareness. In this day of age they don't like being asked market research questions with the fear that the will wind up on some horrible mailing list and be spammed (or even identify theft). So the blurt out the first media outlet that they can think of. This is called "top of mind awarness".
So tracking the ROI for traditional marketing (unless you work for one of the top traditional marketing outlets and then you have a price issue) is not easy.
I think that ROI tracking for Inbound Internet Marketing can be easier then traditional marketing.
Car Dealers are spending more and more on Inbound Internet Marketing and less on traditional marketing. Why? Because some very smart Inbound Internet Marketing companies have come up with a plan that shows the car dealers results.
They set up a landing page with a good call to action offer. The customer must bring in a print out of the webpage and show the salesperson before they start negotiating about a car. The dealer has 100% proof that the Inbound Internet Marketing program is working. And the ROI is a lot less then running an ad in the Globe!
The Inbound Internet Marketer drives traffic to the Car Dealers landing page by Organic SEO or Adwords.
More and more car dealers (and their sector spends more on advertising then any other sector) are taking money away from traditional marketing and putting it to Inbound Internet Marketing.
So when someone ask you the ROI on Inbound Internet Marketing answer by saying. "Sure beats traditional marketing!"

