Driving a 4K display at 60Hz with rMBP
Your question has two parts: whether an rMBP can drive a 4K display, and whether any exist within your price range.

Apple's support site lists compatibility between their laptops and 4K displays.

If your Retina Macbook Pro is one of the 15" models, it can drive 60Hz MST displays at 60Hz through the thunderbolt port.

If it is a 13" model, it cannot - you are limited to 30hz regardless of output.

If your laptop does support it, the Asus PQ321Q is a 32" monitor that accepts a 60Hz image via MST. It is $2000.

There are also numerous 28" models that support 60Hz over DisplayPort 1.2. One example would be the $500 Samsung U28D590D.