I just read this article here:
http://www.popularmechanics.com/technology/how_to/4318204.html
It says a text message is a maximum 160 bytes of data.
Vodafone have a $1 a day for 10MB plan, but they charge 20cents for a text message.
I'm no good at maths, but what is 10MB divided by 160 bytes, because that would equal the number of text messages you can send until you reach 10MB, and I'm sure it is in the thousands, so a text message should cost less than 1 cent to send shouldn't it?