I've recently bought a Nokia Lumia 920 and although my plan at Vodafone NL is pretty large I would like to monitor my data usage. However I can't seem to find any app that does this. There is a little scraper app called 'Belstatus Vodafone NL' but that doesn't work for my plan unfortunately. I looked around on stackexchange and only found this question Any solution for 3G traffic counting? which does not give me an answer and given the date is probably for WP7 anyway.
This doesn't help you (yet), but Microsoft has an application called Data Sense. Data Sense can monitor and limit your data usage. Unfortunately for some strange reason it is only currently available on Verizon Wireless.