WFRP 2e is one of my favourite games with solid rules, great flavour and really nice looking books. My personal highlight of the game is how awful PC's are at the start - you could easily have a party consisting of a rat catcher, a beggar, a bone picker and a jailer - its a nice gritty feeling game where combat is pretty deadly so finding other solutions and using your skills is very important.
The $1 bundle is actually insane. Core rules and the first two parts of a decent campaign (ashes and spires) and a collection of shorter adventures you can drop in. Well worth dropping basically no money at all on, for potentially hundreds of hours of play or even just robbing ideas from.
The $8 bundle is the third part of the mega-campaign (forges), the brettonian sourcebook (arthurian french knight and peasant fantasy) and an adventure in that land (knights of the grail and barony of the damned), a dwarf hold sourcebook and adventure (karak azgal) and a sourcebook/freeform-sandboxy adventure (talabheim).
the $15 bundle is basically pretty much every other book for the game, including expanded magic, rules for playing as skaven. Each sourcebook generally contains either a full adventure or meaty ideas for more as well, so they are worth picking up.
Also, the 20 buck bundle is literally every book you would ever want - more divine magic, more chaos, ANOTHER mega campaign (9 or so adventures long, taking you from peasant to hero of the empire). ALSO includes the 1st ed rulebook which is nice to read and see the changes in the world and system from then to now.
There's a couple of books here that I don't own, so i'm going to pick this up myself. Well worth $20 for like 3 books that I don't think you can get elsewhere.