Miiverse is an awesome stage, background comments aside. Actually, all of the 3 new stages are excellent.
Suzaku Castle is incredibly nostalgic for me as a Street Fighter II fan to see, yet it looks so new and shiny. It's also got a coo layout. That stage is particularly giving me hope that a DLC DK stage would be awesome!
5 dollars for a character is pretty standard in the fighting game industry. I can't speak for stages.
Depends what you compare it to. After you buy Skullgirls, all subsequently released DLC has been free. If you compare it to any Arc System work fighting game or Dead or Alive, it's cheap, but those games have always had overpriced DLC. Compare it to (generally) more mainstream franchises like King of Fighters, Tekken, and Street Fighter, and it's very expensive.
To buy 3 characters (with way less moves than those in Ultra SFIV, and no custom moves either), 2 stages, and 8 outfits in Smash Wii U or 3DS, you must pay $23, or $30 for both versions to make it an equal price.
Compared to the very fair pricing of the Street Fighter IV updates/upgrades (the Ultra digital update was $15, and it came with an incredible amount of new content), and DLC and the Mario Kart 8 DLC (for cheaper in 2 packs you get 50% more courses [generally of a higher quality too] and tons of new vehicles and parts), Smash 4's DLC's price points are total rip-offs. The Smash 4 DLC is 3 times as expensive as it should be, and the Mii Fighter outfits should be sold no higher than $0.20 a piece per game, or $0.30 for one for both games.