It's down to Onebip & the exchange rates. They don't allow some countries to spend over a certain amount, some have a minimum they must spend etc.
Be aware that if you're not in the US you'll be at the mercy of the unstable economy. Some days you'll find that it costs next to nothing in your country, and others the price will have doubled.
Check on a site like
www.xe.com to see what the exchange rate is like (eg today $1 = £0.60), it's the best way to catch a good deal on tc. And don't worry about Nabi, they'll still get the right amount of $ even if it does seem stupidly cheap at times. Infact, why not buy twice as much? ;)