What is new in the 6th edition of Warhammer 40K?
The rule-book itself is full color with expanded art and fluff sections.  Though you won't find anything here that wasn't present in previous versions outside of some new artwork.  Most of the price increase was because it's GW and that's just what they do.  Any other gaming company would probably charge 10-20% less for the same thing.  

GW did just release a rules only version that drops the fluff and color artwork for a price reduction.  The new starter also contains a mini-rulebook just like last edition.

6th was a pretty big overhaul to the game.  For better or worse a lot of mechanics from the current version of Fantasy where ported over.  I'll go over the big changes:


Flyers - Probably the biggest change and also includes Flying Monstrous Creatures.  They move fast and are hard to hit (always requiring a 6+ to hit regardless of BS).  They also neglected to give many of the armies any units with the new Skyfire rule to shoot them down.  This has had a great impact on balance.
Allies - You can take small detachments from other codexes based on an allies matrix.  Some allegiances are good enough that your characters can join up with allied squads.  Game balance has been tossed out the window this edition.
Hull Points - Vehicles essentially have wounds now.  Usually 3 for standard vehicles, 2 for lights like speeders, and 4 for the big guys like Land Raiders.  Pens or Glances take off a hull point.  There is no damage table for glances anymore.  They also made AP 2 weapons add 1 to the damage table, and AP 1 adds 2.  Pretty much everyone's mech has been shelved this edition. It's far to easy to glance vehicles to death.
Snap Fire - Pretty much anything can move and shoot now, but heavies need 6+ to hit if they move.  
Assault - Big changes here.  Assault moves are 2d6 now instead of a fixed 6".  They also included challenges between characters.  Defender can also snap fire into assaulting units now.
Wound Allocation - Another big change.  Wound allocation is by closest model.  Characters can jump out of the way to transfer the hit to a mook on a 4+(2+ if it's an IC).  This effectively allows you to take a nasty character with a big squad of guardsman for +20 wounds.