The amount does not refer to a specific set of millions, so we use the indefinite article. It could/will be ANY milions, not those/these specific ones (about which, we may have spoken earlier, for example).
“des” is an undefined article whereas “les” is a defined article.
We don’t know where do they (the millions) come from, so it is undefined.
“les millions” would imply that they are defined by a context (where do they come from).
- If they come from selling a house: “Ton mariage te rapporte les millions de la maison“
- If they come from winning the lottery: “Ton mariage te rapporte les millions du loto“
If we say just “les million” we consider that the person we’re speaking to knows how they are defined, but it may be ambiguous. That’s why the recommended way is to define them explicitly.
In the general case, money is not defined, that’s why, you say most of the time “des millions”.