Minecraft Modders Will Not Pay A Fee

No way, no sir. That's what was decided soon after Notch posted saying that modders would be required to pay a small fee to register and download the Minecraft source code in order to make mods. The internet disagreed about whether that was going to happen, and so it did not happen.

Other details explain that the mods must not be malicious, or make money, and that Mojang retain the right implement them into Minecraft as they see fit, so as to avoid situations in which the team are forced to copy popular mods, because a mod does something their game does not.