For Players Yes, mod.io is generally free. You create an account, browse mods, and download them to your PC or console without paying a cent. Most content on the platform comes from the community and costs nothing.
However, some games feature a Marketplace. In these specific cases, you can buy Virtual Currency (credits) to purchase premium mods. This supports the creators and the game developers.
For Developers No, it is not always free. You pay to use mod.io as a service.
Indie Developers: You can often start for free within certain usage limits.
Studios: You typically pay licensing fees or agree to a revenue-share model, especially if you use the monetization tools to sell mods.
For Creators It is free to upload. You do not pay to host your files. If the game supports a marketplace, you can even earn money by selling your high-quality mods to players.