1) What is the current market price for a 1.1 pair?
$1200-1500.
2) How rare is this turtle?
Quite rare. It is endemic to 3 Japanese islands. Possibly 3 distinct subspecies or maybe even species.
3) Is it very difficult to successfully breed them?
For the most part, yes. Only 1 or 2 people have successfully bred them in the U.S.
4) Are they easy to keep in captivity? Are they hardy turtles? They are relatively easy. They are generally hardier than G. spengleri.
G. japonica is a bit larger and more robust that G. spengleri and they thrive at slightly warmer temperatures than their congener.
It should be noted that every G. japonica in captivity is illegal. These turtles are a national treasure to Japan. They are a completely protected species and have been for a very long time. The animals that you see in the pet trade are smuggled out of Japan and exported through some other country usually as G. spengleri.