GameZone reports that the Vita won't have any special prices associated with it, and it will instead use the same data plans that they've already been using for other 3G devices, such as the iPad's.

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.