Or perhaps a data block system like Slingshot/Callplus (for broadband/internet services) where you could purchase 240MB blocks for $18 each and stockpile them. What'd be better is if the unused amount rolled over to the next month, but that's another story.
For example, I'm going on holiday at the end of the year to where I cannot get internet but can get signal on my phone. So the plan is to tether my phone to my computer and use IRC and browse quite a lot. 240MB isn't quite going to suffice.