If you're going to be watching them all the time, then it's possible that it could use a ton of data.
The cameras don't really transmit if they don't have anywhere to transmit to. I have a Foscam IP camera watching my driveway, and I get emails when it senses motion. Those emails generally run ~600kb per 3 image series. (HD images). Lower quality images run about 75-90kb per. If you're watching them 24/7, though, the data bills are going to get pretty spendy.
The emails that are sent for alerts don't amount to much without a photo attached. Is/will your data plan be one where you can select the amount of data on a month-to-month basis, so you can fine tune it?