The crazy prices and sudden drops are driven in part by:
Periods of high demand: Bad winters, new markets emerging, want list copycats, and even the high prices themselves spur demand from buyers who see it as a business investment. As a matter of fact I remember seeing an ad not long ago that had no real information about the variety other than it would be in high demand for a long time, which is something that seller cannot even control anyway.
Low supply: With sellers fighting to be near the top of the pyramid (uh-oh) on varieties that have been identified by members of the community (without conflicts of interest and bias, lets hope) as superior AND not readily available, then it takes away the opportunity for some of the more "generous" members you read about to get their hands on it and work their magic.
New information: As varieties get into the hands of more experienced growers the beans get spilled- splitting, shy bearing, synonyms, unflattering photos, realistic comparisons, etc.
Good luck finding a variety that works well in all climates, is attractive, healthy, tasty and not already spread all over the place.
Low demand: Member turnover has increased due to a whole bunch of things: animosity, conflict, reality setting in, competition, distrust, and so on. Average growers cannot justify the prices sellers pay, but they are the ones which lower (on the pyramid) level sellers rely on in order to recover their investments and turn a profit. Wishlists are also a factor here, while the demand is there some people know they can pay nothing at all if they brown nose the right person or support them in a fight or something.
High supply: In the event demand increases or supply decreases and prices raise as a result there are now more sellers waiting to jump in quick and capitalize resulting in a price drop, and more availability throughout the year.