I'm keen to know of anyone's experience/opinion of this lens, which was made for the Graflex XL series.
From what little info I've been able to find out to date, it's reputed to be extremly sharp, and is refered to as a "rare element" (or is that "rear" element?) lens. What exactly does that mean?
Further if anyone is able to give me an idea of what this lens would typically sell for I would be most grateful; is it particuarly hard to find?

A rare element usually refers to RADIOACTIVE materials used in making the glass. Low level stuff, may fog film if left on a loaded camera for several hours, might cause a cancer if you carried it in your pocket 12-18/7.
To test for radioactivity sit suspect lens on an unexposed sheet of film for 24 hours then process as normal (keep film light safe). If a circle is present where the lens was it is radioactive.
Polaroid shet film is perfect for this test.

If memory serves, "rare earths" include nonradioactive materials like lanthanum and ytterbium. They can be added in small quantities to molten glass, changing the refractive index and so giving optical designers more options. It seems reasonable that Voigtlander's "Apo-Lanthar" was so called because lanthanum was used to dope the glass.

The "Ysarex" has an excellent reputation. Rodenstock makes good stuff.