From our tests, a heavy use of iBroker consumes about 5Mb per hour of data traffic. So for iBroker heavy usage we estimate a need of 500 Mb/month.
Alerts are held on the server side and pushed to the device via de Apple or Android notification systems. Therefore the alert monitoring does not consume any data.
To choose the most appropriate data plan for you, consider how many hours a month you are planning on using iBroker, and all the other (non iBroker) uses you will need. Also take into account that if you have access to wifi, whenever you are using a wifi connection you won't be consuming data from your plan.