According to Pandora's blog, new Pandora One users will see a price increase from $3.99 to $4.99 per month come May 2014. Existing customers, however, will stay locked into the $3.99 per month price for now
"Over this same period, the costs of delivering this service have grown considerably. For example, the royalty rates Pandora pays to performers via SoundExchange for subscription listening have increased 53% in the last five years and will increase another 9% in 2015."