We can see that for the last 4 years PV costs about €4.7/W. But inflation means that in the same 4 years the value of those €4.7 has dropped to about €4.17.
A more interesting way to measure the value is the distance from the nearest power line beyond which solar is more economical. I think power lines are about $20k/km. As solar becomes cheaper (or alternatives become more expensive) that distance decreases and more people buy.
The number of people < x metres from the power line is probably quite a steep distribution. So perhaps the reason that the price doesn't fall very fast is that every incremental drop in price increases demand sufficiently to maintain price. When the distance reaches 0 (As it has done in Germany, due to their pricing structure), expect demand to rise dramatically — keeping the price high until that market is saturated (at current rates, taking perhaps 20 years). This is of course pure speculation, perhaps there really is a secret cartel who aim to keep the prices high.