There are two plans at the moment:
250MB of data - $14.99/month
2GB of data - $25/month
Gamezone was informed from a sales representative that doing things like streaming YouTube or music will eat away fairly quick even at a 2GB data plan (though YouTube doesn't currently work with the Vita anyways) but that playing games online shouldn't be that taxing since the game is already predownloaded or running from a cartridge, though it was still recommended to get the 2GB plan if the Vita would be use extensively.
That means you're looking at $300 per year if you decide to go with the 2GB plan for the Vita. A positive point about this is that it doesn't require a contract and is paid on a month to month basis.