PC players bought BG:EE for $20. Let's call that the "full game".
ipad users are given a choice. The base BG:EE game is $10, thats the Enhanced Edition in all of its glory, kits, new UI, native high res and more plus the Black pits and new monk character.
Then you can add the 2 other new characters and the portrait pack for $3 each.
Buying the base game and the rest of the content for the ipad would be $1 less then PC players paid for the same "full game".
Starting with a less expensive base game allows them to have a competitive price point in the mobile platform market. While giving us the chance to get the rest of the content if we want it.
BG was great, BG:EE is better. However you look at it 10-20 bucks for a game that has 100's of hours of playtime is amazing.