Looking at the Vodafone site I noticed that there are some strange combination of pricing.
Take for example the Plan Add Ons page.
Here you can see that, when you get to the 12 month contract data addon sets, you can pay $9.95 for 500MB per month. Or the same $9.95 for 1GB per month.
(Click the Terms & Conditions link under one of the data add ons.)