I think this goes for just about anything. I remember hearing that to increase the quality of red wine by 10%, it will cost 100% extra to produce it. I think this applies to some degree in any product category. However, it does seem to be a radical step up in price from the AK100 to the AK120 assuming that the AK120 is an improved AK100. This last point may be wrong and could be the reason for the jump in price. (Sorry for being off topic.)
Lars brings up a few good points.
With respect to wine, the cost of production of wine and the quality of wine are largely unrelated. Lots of factors are important. However, the quality of the key raw material (grapes) is highly correlated with the growing season weather. A few times each century, people can hope for spetacular grapes. Also the cost of the raw materials (grapes) is vanishingly small compared to storage financing, bottling, marketing, distribution, taxes and the inherent profit taking of each player involved in these steps. Of course vineyards and wine magazines don't provide this info but that is the reality.
In some ways audio is the same. Styling aside, I find the quality of reproduction largely uncorrelated to cost. The assumption that paying more money will drive a corresponding increase in sound quality is flawed. Of course there are bounds to this assertion as a $5 pocket radio likely is so flawed from conception to production that it has no hope out of the gate.
An explicit description of the engineer's design goals and philosophy seems to be a better predictors of "audio quality" to my ears. But not always. This is such a qualitative hobby, I think the best bet is to try as much gear as possible and make your own conclusions about what makes some gear work well. Organize a local meet or visit a big store. If you have the opportunity to spend a few hours in large Tokyo headphone stores that is a great primer. Some companies have generous trial policies that indicate a certain confidence in the quality if their sound.
Back to the topic, the focused design of this player and experience of the manufacturer make for a potential step up in sound quality at the expense of some features. That is a good compromise for me. A trial side by side will tell!