The problem using Internet Data Plan Package is you have to control yourself to avoid over-quota. I'm using Mac OS X Tiger, Sony Ericsson M600i with Indosat 3G Package for internet connection. Monthly charge is Rp 130,000 (promo rate) for 1 GB.
I can check usage in the symbian phone by opening Connection Manager, find GPRS Modem entry (usually created by PC Suite Software), and check its total connection log. How about using computer software to track it? I've crawl so many websites to find freeware for Windows or for Mac with empy hand. One day, Benny gave me a reference to download SurplusMeter for Mac.
It measures your download and upload in PPP port in very handy way. Install and forget. SurplusMeter Agent will run in background even you quit the main program. You can input your monthly quota in the program so it will display usage status compared to the quota, reset the usage on specified monthly date, pause the counter for your own reason, etc. Let this program as your GPRS data usage counter and the best thing is it's FREE under GNU GPL.