Lol..well Nitrous is ok if you don't rape the heck outta it. From what I do know a 50 shot depending on the size of the bottle can range from 3-5 minutes total...maybe more, maybe less. You don't use it that long when racing so it should last a good while...
It's around 500+ new for just a kit...maybe half if you can find it used... depending on who you get to install it could range from 100 to 250 or more. Just be sure you get someone who knows what they are doing.
Try retarding the timing and getting colder spark plugs. Some fuel managment work could be useful too... anymore that 50-75 shot you definatly gotta work the block.
I never took the time to really learn about it so I'm not positive.

Anyone correct me if I'm wrong.