I use the word "conduit" to describe this option because it specifies that the cable housing is routed through a stainless steel internal pipe, custom shaped and brazed into the frame. The progression of Jay Carpenter's build (above), shows some steps in the process of making internal conduit. There are other bikes in the world that have "internal cable routing", except this simply means holes drilled in the frame to feed the cables through, leaving the housings loose to rattle inside the frame, which creates an entry point for water, as well as a nightmare when attempting to change cables and housings.
The internal conduit option is becoming a popular one for my most current builds. Not only does it maximize the beauty of a frame, but also makes a very low maintenance set-up, the required use of full cable housing keeps the cable clean, basically sealed from water and grime.
The price of this option is $150-$300 per cable depending on how many frame tubes the conduit routes through. For example, the internal conduits on my orange bike (shown here), and Jay's bike inter at the front and go through the toptube, into the seatstays, and exit the frame near the drop-outs, so the price would be $300 per cable because it routes through 2 different frame tubes. Another example would be an internal brake line on a cyclocross frame, where the conduit only routes through the toptube. The price for this would be $150 because it only routes through one tube and is considerably easier to build.