Do you have to have the game for PC to get MODS?

Yes.

Yes and no. If you have a modified Xbox, Playstation, whatever, you can use mods with your console games. However, in the US, mod chips are illegal, so you can't use mods with your console of choice.

do the mods cost money?

NEVER. If a mod maker were to charge money for a mod, that would be a clear violation of the game's licensing agreement and the publisher would have lawyers down their throats in a second. If you ever encounter a mod that someone is charging money for, its a scam, don't buy it.

nope free. it would be illegal to charge for them without the company's permission. What game do you intend to mod?
ninja'd
Mods kick ass. Buy half life 2 and youll have like 100 different games to play. Alot of them are shit but alot of them are awesome as well.

Here's a good site to get you started with mods.

What is a mod really?

Strange British youths in the sixties.

a modification of an existing game. Some times they add new content to the game sometimes they are an entirely new game.

If it is a fan-made mod and it is being sold by anyone other than the game's publisher (remember Counter-Strike was originally a mod), it is illegal. If it is legally being sold, then it is not really a mod anymore, its more like a licensed add-on or expansion, like the modules in NWN. Maps are not really mods at all and the last thing I heard about SC2 was that players were going to be allowed to sell maps only.
"Mod" does stand for something. It is an abbreviation for "modification", given the name because the content of the game is modified in some way by the mod's creator. However, there really is no reason to capitalize it.

Ministry of Defense. They suffocate you in your sleep if you say anything bad about the Queen.
Sorry, been watching loads of Spooks lately.

Probably should have specified that it didn't stand for an acronym :p. I just get really annoyed when people all-caps things that they really shouldn't.
And some maps ARE mods, for all intents and purposes. Using SC2 for example: Apparently you can turn the game into an FPS with just the map editor.
Civ4 maps/mods turn the game into a squad-based tactical game.
Don't even get me started on FPS maps/mods :p
And you can sell a mod, it just depends on the terms. Mount&Blade has one or two commercial mods. I don't think they sell well, but Armagan doesn't seem to feel like suing them.
Although, if your definition of "mod" is "Free", then this is all moot.

So they are like cheats?