A properly set up and tuned kit will do no harm to your engine, providing you don't try to squeeze 250 extra horses out of an A12.
I put my kit together for a little over $600. (Including the bottle)
Hollywood has a lot to answer for with regards to the untold myths surrounding nitrous oxide. In reality, you wouldnt use it on the street. (well, not much anyway) And the duration of a shot would rarely exceeed 5-6 seconds. (Where are you going to be able to accelerate flat out for longer than that?
Some say why would you bother, I say whay wouldn't you bother? Some is good, more is better, and too much is just enough!