It certainly costs a fortune to use reguarly, the trick is to make sure your car is quick enough to keep you happy 99% of the time, then just use the nitrous to top it up for the rare occasions you need more.

For example i have a car (cant mention what im afraid) that had a 2.0 16v engine with about 150bhp, and i had over 100bhp more power on the gas, but due the vehicle having masses of traction etc i found myself always wanting the gas on, and was easily getting through a couple of bottles an evening (at nearly 15 quid a bottle), so instead i am now half way through swapping that engine out for one that is 250bhp (will be nearer to 280 soon hopefully), which is ideal for normal road use, and then the 100bhp nitrous kit will only get used occasionally as a top up.