I'm trying to find the best-value option for mobile data on a tablet; I'm not needing a huge amount - I'd say a minimum of 500MB/month, but ideally say 1GB.

Given the Skinny pre-pay plan now gives me 1GB of data plus calls and text for $16/month, the prices for data-only options offered by the various telcos seem on the high side!

At the moment the cheapest option appears to be with Vodafone: buying $20 credit for four months, which gives 500MB data for free for each of these months; then using the $80 built-up credit for 5GB data for six months; this works out at an average of $8 a month, which gives means 500MB/month for the first four months and an average of 833MB for the following six.

Are there any better-value options out there? 

Appreciate any advice.