How much HP will turbo add?

I'm curious to know, in the realm of automotive performance and specifically turbocharging, just how much horsepower (HP) can a turbocharger realistically add to an engine? I understand that the increase in HP varies depending on factors such as the engine's displacement, compression ratio, and the specific turbocharger model and configuration. But in general, what kind of power gains can one expect when installing a turbocharger on a typical gasoline-powered engine? Is it a significant bump, or just a modest increase? And how does this compare to other performance-enhancing modifications like supercharging or nitrous oxide injection?