Depends on your server host.
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Mods are free. Just download them and get them into your minecraft.jar.
It costs nothing to vote for servers
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)
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.