How much data do iMessages use?

Very very little if its just text messages.
If its pictures or videos then the amount of mb's the pic or video size is. It sends it through with the actual size it is on your device without shrinking it.

 

They use roughly a byte per character - If you do the maths you'll find it's a minuscule amount, that you'll be able to send millions of text messages over iMessage with even the smallest of data plans Regular text messages use the same amount of data, however phone companies overprice them horrendously, as texts are their biggest cash cows.

 

So very small. Literally like less that a KB

 

Well when it comes to sending gifs - it can be a lot. But most the time it's negligible.

 

Megabytes per message. They are constantly using data to see if your buddies are online and typing messages to you.

 

Even so, it's still negligible - The amount used would be sooo tiny, even for heavy texters. It's only when tons of images start getting send across that it uses a significant amount of data.