The game will have currency that can be used for purchasing items. You can find emeralds and other gems inside crates, which you can find all over the world and break them.
That is why it's so important to explore every single nook and cranny, because you never know what you will find inside. In this way Mojang will encourage players to explore the map as deliberately as possible.
In-game currency like gems will be the only real currency in Minecraft Dungeons and there will be no microtransactions.