For a single user I would start pricing at $5/m on an annual subscription or $9 for monthly. Getting customers in the door and retaining them through the early product evolution cycles is the major hurdle in the early stages.

Beyond that, I would create an offering with 1-2 features tuned towards business customers. This kind of tool is a great knowledge management addition, and knowledge sharing is hugely valuable for business customers.

I'd price it the same per seat at the start ($5/m annual vs 9$/m monthly) and sell multiple seats at a time this way. If this works well, you will have found an existence proof of a highly lucrative customer group.