A box of blindbags is 24 ponies, one of each. There's not really any such thing as a "rare" blind bag, besides the pink Twilight Sparkle that was a prototype in wave 1.
Some of the mane 6 are repeated in waves, (though some end up getting different-colored plastic in subsequent waves), so they're more "common," in that there's more than one way to get them.
The value of a blind bag is determined by how popular the character is. Before wave 1 hit America, plain Fluttershys were selling for hundreds of dollars, and now I'd say she's worth about a dollar.