I'm from Europe and for me, the price from cardmarket is a crucial information during deckbuilding, while the currently shown prices have little-to-no value to me. Was there any thought on adding this as another column in the deckbuilder, or even allowing users to select between different price sources in their profile? Or does someone have an solution for this problem? I would even be fine with this being a premium-only featuyre.
Other sites, like Scryfall or edhrec, show these prices.

This is one feature that I truly miss. Deckbox used to pull from tcgplayer, however it lost that ability once it created its own marketplace. This happened years ago now so there is a chance that you are not aware of it. I don't foresee deckbox being able to get permission from any other site because of being seen as competition.

As a client-side workaround, i made a tampermonkey-script that shows cardmarket prices for deck lists (in the table itself):
https://gist.github.com/fakusb/29ab3168a8bca1bb8e87982da5b95531

As this script uses Scryfall (and they have not only cardmarket prices), just replacing "eur" with "usd" everywhere in the script should show tcg-player prices instead.

Just don;t refresh to fast: Scryfall does not like more than 10 requests per second. I cache prices for 1 day, but jumping between 'uncached' decklists too fast might overstretch their generosity.