a typical text message will be less than a byte of data. The lowest end data plan in america right now offers 200MB / Month. So if you only used that for iMessage you could send over 200 million messages. Are you really that concerned?
Just here to correct your math.
One Byte of data in normal computing terms can only represent one character.
I wish we could actually break down how much data is actually sent/received per iMessage.
1 byte per character in the message
+ packet headers (minimum packet size?)
+ polling the servers to see if the recipient is online
+ delivery receipt confirmation
+ read receipt confirmation if the recipient has it turned on
I just turned off my cellular data, reset the stats, turned on cellular data, and sent 2 small iMessages to my wife. (20-30 characters each)
Usage after the first was 18KB sent, 32 KB received.
After the second it said 42KB sent, 55KB received.
It's not scientific proof, and my iphone might have been doing other data requests.
But it seems to use a LOT more data than we would think.