I've been using 'My Data Manager' for a couple of years now (I think), and battery usage is not an issue. It's never registered notable battery usage on my HTC Wildfire S
I've previously sugggested that if your on the £10 Goodbag with the 1GB data limit then it an absolute must to have a data manamgent app. 'My Data Manager' was the app I suggested. I've tried several and it's the best I have found.
A very useful part of the app is the ability to set 'Usage Alarms'. This is vital if your on a limited data package.
I have only 1GB so I have set the alarm to notify me when I hit 300MB and 800MB, and then turn off data usage at 998MB.
Ironcially this post comes on the same day as a new update for the app. I never update apps before reading reveiws of the update on the Android Play Store. It appears the new update may not be as good as the old version, because the developer has changed things to make it more fashionable with minimilist appearence which provdes less info. I'll wait a few more days for more reviews before deciding to update. Regardless of that I strongly recommend this app, and it is a must if you're on the £10 Goodbay with £1GB data.