Here's how it works (as I understand it), assume that you originally start your plan on January 1st regardless of your original choice:
Option 1: Buy the Unlimited plan.
$30 / 30 days
Option 2: Buy the 250MB plan, and use less than that every 30 days.
$15 / 30 days (your second payment is on Jan 31, your third on March 1)
Option 3: Buy the 250MB plan, and use it up every 20 days.
$15 / 20 days (Your second payment is on Jan 21, your third on March 10)
Option 4: Buy the 250MB plan, and use it up in 10 days, the upgrade to the Unlimited plan.
$15 / 10 days, then $30 / 30 days (Your second payment is on Jan 11, your third on Feb 10, your fourth on March 12)
In this case, you underestimated your requirements.