I feel your pain. I spent the past few years on satellite and it was very unpleasant.
Your best bet for monitoring and controling your Internet usage is buy LittleSnitch. It will monitor your Mac and tell you every time that it is trying to make a connection, allowing you to allow or deny the request.
For some apps (such as email, web browser, etc), you may want to allow access anytime it wants it so you don’t have to approve each time.
The first few days of using LittleSnitch will be frustrating, as you start to set rules and tell it what to do, but after you get past that, it will be a big help in keeping your usage in check.
Unfortunately OS X is not very good about telling you everything that it is doing, and it assumes that any available Wi-Fi connection can be used without limits. Until recently iOS was much better about this, but it now allows much more background activity, so if you have an iPad and/or iPhone/iPod touch you need to watch out for those as well.
When you are not using MacBook, I recommend closing the lid so that it will sleep.