As you see on the pricing page, the price depends on the number of developers using PullReview. It does not depend on the number of repositories or the number of reviews. The reason is simple: PullReview's operational costs are due to the infrastructure needed to do the review and store them. The more reviews generated, the more it needs power and storage. It is as simple as that. The question is then, what triggers a review? A commit made by a developer. So the review activity is directly linked to the developer activity. That is why the price is linked to the number of developers.
For the price per developer, it has been mainly chosen according the added value of PullReview, the market, and our operational costs. If you compare the price with the minimum wage of a junior developer in US, it corresponds to one hour of work. PullReview will save you more than that, whatever your level.