(This is USD, I don't know what is charged in other countries)
I think there should be an option for 10GB for $5 or even 5GB for $2.50. Its the same amount but a lot more people would be willing to pay smaller amounts so in the end it'd increase profits anyways,
I know that personally I'd be more likely to boy $5 worth of beetles several times a year rather than a larger amount only once or twice a year.
This would especially be good for minors and unemployed players.