Well, it's the first ever made "green star" lens, so there is a special collector's value to this lens. For usability it lacks the A-setting, so it will have to be used as any M or K lens, but it should be as good as the A* 300mm f4 Harriet has. And that is one goooood lens.
Seems to me like the prices for used Pentax lenses vary a lot from country to country (strangely, since ebay is worldwide), and from time to time. Without scientific research my feeling is that the prices have started climbing again after a period of recess. Myself I only buy if I find a bargain price, but then again I'm not in deep need of anything so I can be patient.