In game design, a cheap entity/effect/function is one that can be processed very quickly at run-time. As an example, in Source, cheap to draw objects are displacement surfaces, brushes and decals.
Many cheap things can be drawn on-screen/simulated in the world at once, but they are generally of low visual or computational quality. Decals and other 2D images are examples of cheap objects that do not (necessarily) lack quality.
The term is also used in reference to relatively cheap processes, when more than one is available. Such as water that uses a cubemap instead of proper reflections.