There are quite a few: PP 130, 530, 533 ... 1579 ... 5070 ... 5960 ... 5950 ... 5980 ...
Why so expensive? Different answers, depending on the watch - rarity, beauty, reliability, le confort, ... - a well balanced mix out of those.
To understand the complication of the chronograph, one has to go in detail - column wheel vs ..., flicking minutes, synchronizing clutch and brake, finishig ... .
To understand the chronographs from PP is even more difficult, because from my point of view there are at least three levels and they are produced from different people in different departments.
By the way, there is no special complication department for annual calendars. Just food for thought!
If there are no differences for you, you should not spend money on it. No offense, just honest!
Oliver