The agents have done good work in pressing publishers to get subscription rates out earlier than they used to do. Unfortunately, there is a downside here in that the information used in the pricing process is bound to be less reliable than it would be later, and any prudent publisher will tend to assume a worst-case scenario if confronted with a range of possibilities. But this is a relatively minor matter. For pricing purposes publishers look at three items in particular. They work out costs, they look at attrition and they project other income.
Costs are mainly production costs.
In spite of what some librarians seem to think, there are very good reasons relating to the health and success of the journal for increasing the number of pages, and thus the costs and thus the price. Publishers also have to think how they factor in not just the extra costs of producing e-files but also the costs of supporting the infrastructure, which is a relatively new development. Costs are not however my main concern here. Nor is non-subscription income. It is not a really significant factor in academic journal publishing except in some medical areas.