Mods (modifications) are free. You do not need to pay, however, you do need to be able to download them. Use Magic Launcher or Forge to do this.
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Mods are free. Just download them and get them into your minecraft.jar.
Depends on your server host.
Modloader, Modloader MP, and most Minecraft mods are completely free. You can download most of them at the Minecraft Forum.
up to this point, i've never heard of mods costing any money? they're all free, as long as you have minecraft installed (which you can also get cracked for free)
The hexxit mod is free. It is actually one of the better and safer mods out there.