Similar to Adding Minutes, Messages or Roaming Add-On's A Data Booster Add-On Package would be nice if you know you will be going over your limit.
They would have to be at least slightly cheaper than the base charge and could start at 500MB then to 1 GB then 2GB, 5GB and so on all the way up to unlimited if need be. Because sometimes you just need more data.
You could also make each incriment slightly cheaper than buying multiple lower increments. For example Buying 2GB is cheaper then 2X 1GB Packages or 5GB is Cheaper than 2X 2GB and a 1GB Package.
To Summarize the Data is Cheaper than it is when you go over your current plan usage but if you are not sure if you will need 1GB or 2GB you feel better buying 2 because it will be sligtly cheaper than 2X 1GB Packages.