Microtransactions in Starfield are summarized as...

The game has a deluxe edition which is $30USD more expensive than the base game. This edition offers nothing but additional digital rewards in the form of an unannounced and unknown DLC, character skins, digital art book and soundtrack, and for many most importantly; 5 days early access.

Additionally there is a special edition priced at $300USD which includes a (physical) watch in a premium case. The details of what this watch can do, what it's made of, and generally what it is are not yet known but it is available for purchase.

Pre-purchasing any edition of the game grants you access to a handful of character and weapon skins.

There are no other forms of monetization announced as of yet.

You could pre-order DLC at a time when no details were known about that DLC.
You can (or could) gain early-access to the game by spending more money.
Features unique cosmetic rewards for pre-ordering the game.