Some fellow named Michael Moffitt (whom I knew from another website) found a way to manipulate CRT menu overlays to display a 15khz RGB picure. See: http://mikejmoffitt.com/articles/0032-tvrgb.html. I was going to try to do this on my small '98 Trinitron (I know what not to probe and how to discharge a tube so as to not fry my Atria) but managed to screw up my Genesis (which was the easiest source of RGB for testing) trying to pull RGB from the board. I'm pretty sure it's a faulty cart slot connection, though, so I have to goof around with that some more.
How much cheaper can it get? NESRGB boards are around that price, and do more or less the same thing. It seems that in order to get any sort of output beyond S-Video (with its low chroma resolution), the GITA will have to be emulated with an FPGA.