In general the ROI (Return Of Investment) is calculated as (gain of investment - cost of investment)/cost of investment, which gives a good measure of what you get compared to what you spend. As you probably already know, the higher ROI the better.
In your case, you have a knowledge database for computer problems solving. As you also note in your question, you don't do any marketing activity, therefore there is no straightforward way to measure the gain of the investment.
One key point to mention is the buying cycle which explains the 5 steps to convert your site visitor to a customer. Aw****areness: Through your SEO people find your website and get to know about your products and your services.
R****esearch: After getting on your site, people can search for more information about your product/service, and refine their search.
Preference: This step contains the process of making the final choice and the confirmation of it.
Purchase: Now that the final choice is made, the visitor can be converted to an actual customer. Loyalty/advocacy:.After the purchase, we expect a satisfied customer that will come back to the site, and search for other products/services that are provided.
The last step takes us back to the first one, as we make a customer aware of more products.
As you can see, the key point is the marketing, that you make people know what you "make" and try to convince them that they need it. That is the main aspect of ROI.
On the other hand, I noticed that you don't have anything related to the social media (facebook like and share, tweet, etc). That is the best tool in our days to attract people and let them know what you can provide. In fact it is also a very good way to boost your ROI. Even in your case, it would be imperative to have people to able to share their experience on your site, for example: "I was very satisfied by this article" along with a link of a solution shared on facebook or after contacting your live support to share that "The article didn't work for me, but they have a great online live support!". This will bring more people on your site searching for answers and/or solutions.
If you would like more information, check also these links:
http://www.quicksprout.com/2014/05/16/how-to-calculate-the-roi-of-your-seo-campaign/
http://www.quicksprout.com/2014/06/27/how-to-calculate-the-roi-of-your-social-media-campaigns/