February 2021
Some modders have patreon or other means to offer monetary support for their work. But if EA were to sell the stuff, it wouldn't, by definition, be a mod.
March 2021 - last edited March 2021
@myosithuhtun Mods don't get sold. In fact, EA has released modding guidelines for the C&C franchise that forbid that.
This means that any mods you can find for the C&C games are simply 100% free.
As @ValFreya13 said, if a mod would be sold by EA, it'd simply be an official expansion / DLC, not a mod.