Nitrous is an oxidizer.. You could throw sparks at nitrous all day long and nothing will happen.. The heat from an existing flame and fuel source splits the nitrogen and oxygen atoms, freeing up lots of oxygen to make the existing fuel source burn better..
It costs about $35 to fill up a 10 LB bottle here. You don't ever use all the nitrous in it, so on average it takes me about 8# to fill the bottle up
I know about that, just talking about a compressed bottle. Not that it will explode in a fiery type, but a puncture isn't gonna be good. I worked at fed ex and someone mailed a compressed air bottle, a forklift driver hit the box on accident and had this box shooting around, quite funny but would hurt if it had hit someone.
Quality solenoids and switches are expensive. Nitrous kit is not a good place to cut the budget IMO. That said, $700 should get you a nice kit with a purge.
What is the $700 for? Well, go down to a place that fills bottles and have them just spray the gas all over your car and see how well that works. I don't understand the confusion.