I'm dealing with this issue as well.
With the same apps installed I usually used a 524 MG data plan. Then I upgraded to 1GB data Plan and now -from several months until now- it doesn't even last for a month.
On top, NetStat is not even working -apparently just for me!- anymore, so I cannot know when my Preł is getting more busy on the net.
I haven't installed any app that requires much data usage, so I can't explain why my data plan keeps falling short about 10 days from date line.
I would love if somebody gets to solve this (probably an app that could measure data plan usage per each app).