No, because iCloud won't backup your device unless you're connected to Wi-Fi and plugged into a charger. So it's Wi-Fi and battery conscious.
I have a 4GB a month limit, with tethering/personal hotspot, and I've been running iCloud since the first beta. It's definitely not going to use a lot of data, not a noticeable amount at least. Now, if it backed up every single time you plugged it in, 3G or Wi-Fi, you would definitely end up with major data usage. Some of my backups have been 2 - 3 GB each. I assume this is why Apple made sure it only backs up on a Wi-Fi connection.
Hope this helps!