I can pretty much guarantee it won't be like that though. As others have pointed out, the arx price will be slightly more or slightly less than the size of an arx pack, so you'll always have some left over (encouraging future spending because you're "already partway there"). These methods are tried and tested, and they work. There would be no point in FD pursuing this if there was going to be a straight 1:1 correlation between the old and new costs.
It's even possible that the effective price of a given cosmetic will be slightly less than it currently is, in raw arx-per-£/$/€ terms, making it look like a bargain. But it will be the pack thresholds that will get you over time. For a while I thought there was a small chance that FD might do things differently, and that their priority was indeed harmonising the payment system across platforms. But the second they confirmed that arx would only be available in packs, all other bets were off. There's only one reason you do that. They'll punt it as a convenience, but it's psychology.
To get really cynical for a moment, this may also be the reason gifting of arx won't be a thing. Gifting someone the value of an item won't be practical if the prices and pack thresholds don't align. And if you allowed gifting of arbitrary quantities of arx, players would organise outside of the store and arrange to trade and pool their spare arx, removing the psychological incentive of having those extra arx sitting there.
As for arx-through-gameplay, until we know the variables (gamplay requirements, arx-per-unit time, whether there are limits on maximum arx you can earn or maximum arx-per-unit-time you can accrue) that's not even a dangling carrot. It's just someone having used the word "carrot" and set everyone salivating like Pavlov's... erm... rabbits.
So, you have a bit left over. Then that goes towards the next purchase, perhaps you will make up the difference through in-game ARX.
The only way i can see this being a problem is if you make one purchase only and never again make another purchase. I think that is pretty much edge case.
I mean, you can almost guarantee there won't be such granularity you can always purchase exact amount. The only way FD could do that would be to offer packs of 1 ARX for sale so you could make exact amount purchases.
So they will offer (for example) 500 for $5, 1200 for $10, 2600 for $20, and you buy whichever you want, keeping in mind you get better value for money with the higher packs and consider your budget and your expectations for purchases in the future.
If you need 600 ARX you might get the 500 pack and earn 100 in game, or just buy the 1200 pack and have the remainder left over for your next purchase.
In Fortnite i can make their currency, vBucks in a number of ways. Obviously by purchasing packs of vbucks with increasing value the more you spend. Through Battle Royale by gaining levels (with battle pass only, cost vbucks), or through Save the World missions with vBucks, daily quests, and occasionally other quests.
Plenty of routes to get the currency without spending any real cash.
Isn't all this just a storm in a teacup. At the end of the day things will be cheaper as long as you play the game. It will be easy to evaluate the cost of paintjobs or whatever using the base package currency as a baseline and in-game activities lower the cost.
Example:
Asp Explorer ship kit currently in store for $10.
ARX price is 1000
Cheapest pack of ARX is 600 for $6
So you spend $12 on 1200 ARX
You spend 1000 ARX on the ship kit.
You now have 200 ARX left over worth $2 for your next purchase.
Equivalent cost of ship kit is $10.
How do you know if FD are creeping up the price? By watching the price for ARX and the cost of cosmetics in ARX. If they go up, then you know things are getting more expensive in real money.
Also, maybe this has been answered and i didn't see it, but i don't see anything saying that the Frontier Store would do away with making direct purchases for cash or that they would no longer display the cash value of cosmetics. They said that it would replace the Frontier Points system on consoles. So, if this is the case, then people will still be free to make direct purchases using cash, presumably though no mixing/combining ARX and cash payments.
If they did mention this, please provide the relevant quote.