I live in the U.S. so I cannot comment on the German Lego market. That being said, the answer to your question must lie at least in part to the European market for CMFs and not in the inherent price of each mystery bag.
What I mean is the price of a CMF in the U.S. has remained at a steady $3.00 since Series 3. While distributions within boxed sets have appropriated the inherent value of the figures above or below that price, a lucky person could still theoretically get whatever figure they wanted for the standard price (if they are as crazy as I am and feel up all the bags in the Lego aisle.)
Now please correct me if I am wrong on this, but I believe the value of the Euro has depreciated in the past several years. Certain principles of microeconomics may maintain local purchasing power even through this drop, giving the illusion that funds go just as far. Increases in the amount of paper that goes in a wallet at the end of each pay period may even make it seem -- on a local level -- that a person is actually making more than they used to, when the extra amount is due to adjustments in inflation.
Now the value of a CMF as stated before has not changed, but perhaps the numerical price associated with that purchase may adjust for inflation. Could this be the reason it appears that the theme has grown more costly over time? If not, please tell me I'm just a nosy American who shouldn't be commenting on European economics. You won't hurt my feelings.
Bonus content: here are the apparent values of minifigures thanks to uneven box distributions, as well as a link to a very useful forum topic.
Hope this helps!