lchiu7: As the OP I know if there were a $20/1GB/60 days or $10/500MB for 30 days I would buy it. But I can see that there is potential loss in revenue for those who might be buying 1GB and only use about 600MB who might switch to the 500MB plan and manage their usage more carefully.
Yes, I agree, there isn't much incentive for 2Degrees to have a $10/500MB. I think if the pricing structure was right, e.g. $15 for 500MB, then there could potentially be more uptake from users who use more than 100MB but less 1GB, and the customers who are roaming on Vodafone and can only get $10 for 100MB won't feel ripped off. Those who use >500MB will still stay on the 1GB addon.
2Degrees could then use that as an opportunity to shift casual data users to higher usage data users. For example, one year ago, 100MB was enough for me. Now, I'm using between 500MB - 1GB. All it took was having more data at cost effective prices and having a smartphone and I was hooked.