Iphone prices will be first/ website second (prices may be different depending on your country.
20 credits: 0.99/ Can't buy
50 credits: Can't buy/ 2.90
120 credits: Can't Buy/ 6.90
250 credits: Can't Buy/ 12.90
450 credits: 19.99/19.90
1000 credits: 44.99/43.90
So until 220 credits it is better to buy credits (by 20 only) on iPhone.
Seeing how 120 credits is only 5.94 on iPhone. You save close to a dollar. However at the next increment it is more worth it to buy on the computer.