Psyonix are wrapping up the work on implementing a loot box free shop in Rocket League and RL Items they released a blog post detailing how it will work, including the virtual currency pricing and key conversion rates.
Rocket League players are likely the notice the lack of the Showroom first, once the shop changes kick in. This part of UI will be replaced by the Item Shop, which can be accessed from the main menu.

The Item Shop will two main ways of selling things - through Daily and Featured items. As the name suggests, Daily item offerings will rotate on a 24-hour timer. Meanwhile, Featured items will have 48 hours before disappearing in favour of the next set of items for sale.

Both sections will offer a variety of in-game items that include painted cars, player banners, exotic wheels, goal explosions and more. Each of these items will have a set Credits price. In case you are not entirely sure yet - yes, that's the virtual money that will be paid for by real-life currency. Psyonix also revealed the pricing of the currency, as listed below:

500 Credits - $4.99
1100 Credits - $9.99
3000 Credits - $24.99
6500 Credits - $49.99
All the prices listed are in USD but the states pricing in other parts of the world will be the "regional equivalent" for the US currency. It remains to be seen if regular conversion rates will be applied or if, for example, European players will simply pay €4.99 for 500 Credits.

Since loot boxes will be gone, so will the keys. Once they get removed, Psyonix will convert the keys into virtual currency, at the rate of 100 credits per key.

Keep in mind that all premium DLC will be gone once the store gets updated. The items Buy Rocket League Credits
from the packs will be temporarily gone and they may return individually through Daily or Featured item lists.