I expect rMBP prices to remain the same or possibly even increase.
The problem with Apple's current lineup is that there's not much separating a low-end 13" rMBP from a high-end 13" MacBook Air besides the retina display. When all Apple's laptops have retina displays, what differentiates the two 13" models in the mind of the consumer? Minor specs like slightly better integrated graphics aren't something you can market.
The hypothetical new 12" MacBook seems like it's designed to draw a dividing line between the two classes of MacBook - the Air line for ultraportability and the Pro line for prosumer/professional work, just like it used to be. The 12" will likely replace both 11" and 13" Air models and be priced accordingly, while the rMBPs will have significantly better internals as well as several ports (USB, Thunderbolt, HDMI etc.) that Apple can market as "pro features" worthy of a higher price.