[WJW]

Mostly depends on the volume of the part, or equivalently it's weight. Complexity you get mostly "for free" when it comes to 3d printing. What type of material you need to "withstand temperatures and pressures of a rocket engine" is entirely dependent on which part of the rocket engine we're talking about. A fuel injector has radically different requirements than a supporting strut for example.

3d printed titanium goes for 300-400 USD/kg, steel is a bit cheaper at ~150 USD/kg for most inconel grades.