Ninja Gaiden 3 is being bolstered with DLC today, including new weapons for both single- and multiplayer. The "Falcon's Talons" weapon from Ninja Gaiden 2 returns for use in the campaign, bundled with an increased level cap and new stage for multiplayer. This is a free download distributed as an automatic update.
Also free when it arrives on April 24: the "Ultimate Ninja" difficulty setting, which makes the game even harder to get through ... although it's kind of hard to get through now for other reasons.
If you absolutely must pay for DLC, you'll be happy to hear that new Ninja Trials packs for each difficulty, unspecified "customization items" and Metal Claws for multiplayer will all be sold separately and as part of a $10 "Ninja Pack 1."