I'll never spend another cent in the store unless the amount of Arx received in a package is equal to or perfectly divisible to the Arx cost of the cosmetic. Example: If a paintjob is 6k Arx but the smallest packages only come in 5k or 8400 Arx.... then Fdev can kiss my and I'll just milk free trickle of Arx per week.
I believe that a paint-job costs 100 Frontier points right now, and the conversion factor is 20, so 2000 Arx*. If you buy 8400 or more then you lose 25p per £5. I'd call that reasonable.
The whole reason to have this currency is that it isn't possible to make small real-world transactions because the bank takes all your money. So you need to buy the currency in large batches. If everything cost the same, then they might be able to make them divide out, but then there's no incentive to buy large numbers of Arx, and large batches mean lower bank charges and more profit. Also you can't vary prices so much. I'd hope a decal was cheaper than paint, and I know COVAS is more expensive.
We're all going to have odd bits of Arx lying around anyway, because we're earning them by playing the game.
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* It also means that a paint job costs £1.25, which is a significant price increase over the current 6-pack price of 50p, and makes it 5 weeks to earn a free one.