Major vendors post projection reports to assist libraries with budget planning for the coming calendar year. See:
While Ebsco rightly calls its report "Projections," Swets accurately labels it "Increases." Has there been any projections for lower serials prices in recent memory?
You might also want to check out Library Journal. Every April, LJ provides both data and current information about concerns in the journals marketplace that might have an impact on prices in the coming year. Although the title changes, this feature usually contains at least the words "periodicals survey."
It is best to look at all three sources, which tend to differ to some degree. That is not surprising, because they rely on different sets of data. Used together, however, these reports do furnish some degree of certitude in the uncertain future of journal pricing. (While we generally know the direction of pricing, we need hints about the steepness of the climb.)