I don't think GBA games are available for sale yet. --- I'm clever, vaguely feminine, a vampire, and I wield Dice. PH33R. The 3DS and PSVita are both outstanding systems. Wally the Equality Weasel says so.
If you bought a 3DS before the price drop you got 10 free gba games and 10 free NES games. Most of the NES games are now available for sale, but the gba games are not available otherwise. The gba games for those that have them are lacking in features found in other vc games (no save state, no sleep mode when you close the system, and the game closes if you return to the home menu).
As for prices, NES games are 4.99, Game Boy games vary at either 2.99 or 3.99 depending on title, and Game Boy Color games are either 3.99 or 4.99 (except for Link's Awakening DX which is 5.99). There are also 3 Game Gear games at 4.99.