What is the computational cost of rendering every additional bounce of lighting?

Technically, inf number of lights can be bounced from a surface. Generally for raytracing, you can specify how many samples per pixel you want (how many rays of light you shoot to a pixel), and how many bounces you have. You can see this is a exponentially growth. There are ways to reduce the amount of computation (russian roulette and etc), but still it is a trade off between quality of image and rendering time.