How much does an iMessage cost on Mobile data?
To answer your question as best as possible, lets look at the major points of where the data fluctuates. You didn't say if you had an SMS (Texting) plan with your carrier either, so if you do, just use that.


Media messages with pictures/video will be considerably higher than a basic message.
A basic message is kilobytes in size and very small.
iMessage was developed as a free messaging service between Apple devices via Wifi networks which wouldn't count toward you texts per month/data plan. 


So the answer to how much it will cost you is not available from the AskDifferent community without knowing how much you'll be using it and what for.