Dark Crusader's discard 1 Dark monster to gain 400 ATK effect can been activated multiple times right?

Also just to clarify this, does his effect last until the end of turn or is it permanent?

I said that it was till end of turn while other people are trying to say it is permanent boost. Usually cards state until end of turn or until the end of your opponents turn so I figured I would ask the question.

Correct. It is an ignition effect, so there is no limitation to how many times it can be activated during your Main Phase 1 or Main Phase 2, as long as you can pay the activation cost.

Dark Crusader is a permanent boost:

Wow, thank you, at least now I know that if I wind up with both Malicious in my hand at the same time, Dark Crusader and no D-Draw I can summon Crusader, ditch 2 Malicious to become a nice beater :)

Does Crusaders effect work with Dark World monsters?

No..........................................

ack! ch-oking on....cost!

Well, not only that, you have to remember that send =/= discard.