An XFI-SFI interconnect runs up to 10.3 Gbps on a single serial link. It is double-pumped (bit on each end of the clock) so the clock rate is half that. This is the connection that links a 10Gbps phy to the transceiver module. You do have to keep the interconnects pretty short though.
http://www.altera.com/technology/high_speed/protocols/10gb-ethernet-xfi-sfi/pro-xfi-sfi.html
XDR ram can transmit 8 bits per clock on a serial line: http://en.wikipedia.org/wiki/XDR_DRAM
Also, note how this is not a single serial 50 Gbps link - it's 4 parallel 12.5 Gbps links. You can run light in parallel with no interference, the trick is to make sure that each independent channel uses a different wavelength instead. So, they are doing it in parallel. Some 100 Gbps ethernet standards use 10 parallel 10Gbps lasers running at different wavelengths, but they are amazingly expensive because of this.
It is definitely not hundreds of times more - here, you can compare every manufacturer for the last 20 years.
http://www.npr.org/templates/story/story.php?storyId=124235858
Remember Darwin; building a better mousetrap merely results in smarter mice.