While announcing the September games for PS Plus (seen below), PlayStation also announced via the PlayStation Blog that the prices of PS Plus are increasing.
At the bottom of the blog post it is said that beginning on September 6th, PS Plus pricing will increase as follows:
This change obviously represents a significant price increase over today’s prices as it equates roughly to 35% at each tier. As of this moment, PlayStation has not announced any changes to the services themselves. We will keep you posted if that changes and obviously discuss this on Sunday’s Bitcast!