Microtransactions in Forspoken are summarized as...
At release there is no purchase other than a deluxe edition. This deluxe edition contains an upcoming DLC (release date unknown), digital art book and digital soundtrack. At 25USD/EUR extra this is a steep price, regardless of that the unknown DLC will be and cost.
Purchasing this Deluxe Edition ahead of the release of the DLC will grant you early access to this DLC. The DLC can not be pre-purchased separately from the other deluxe edition benefits.
Pre-purchasing the game earns you a handful of cosmetic items for your character. The developers have noted that these cosmetics will likely be available for purchase separately at a later date. It is unknown if there will be other cosmetic items.
The game is very expensive across the board, but notably extra so in Europe. The base game sells for 80 EUR, where as other AAA titles sell for 60 EUR or in some cases for 70 EUR.