The smartphone/tablet games are so cheap because for the most part, they aren't really games. That or they have a pretty poor overall quality. I'm not saying there aren't some amazing mobile games out there (Infinity Blade comes to mind), but the overwhelming majority of them that I've played/seen are pretty piss poor.
And if I remember correctly, Game Boy games cost the same as games for NES/SNES, or almost the same. Correct me if I'm wrong here. But why should a handheld title cost less if the same amount of time and effort was spent during the development of the game? The Vita is about as powerful as a PS3 (from what I've read), so I would assume that games would require a similar investment, meaning they would need to set a similar price point to maintain profitability.