The Ninja Gaiden series was always very fun, wasn't it? So doesn't it make perfect sense that pirate makers might want to spread that fun among those who couldn't afford the real cartridge. See, those pirate dudes always have you and me in mind, even when they made this pirate cartridge: Ninja Ryudenken III.
Expect to pay anywhere from $3 to $40 for a single-game pirate cart, depending on which game is on it. This one shouldn't go for much, simply because it's very common.