I would doubt that a credit card reader would use too much data. You can install an app like 3gwatchdog which will keep track of your data use to find out exactly how much data each transaction would require. Some of the apps that use the phone's camera like a barcode price scanner seem to use about 1/3 of a meg, but they are also having to display a picture.
I found the following link which shows that the device only uses from 10kb to 50kb per transaction.
Mobile data is good. Life is too short to seek wifi just to check email!