Zex Kit 82015: Dry kit with jets for 75 to 125 HP-- $450-$500
I would also recommend the following to go with it:
Nitrous Guage: $200 (Electric)
Fuel Pressure Guage: $200 (Electric)
Bottle Heater: $130 (Pressure Regulated)
255 L/Hr Fuel Pump: $100
Guages are not required but are a good idea to monitor the system, especially with a bottle heater! The heater is not required but unless it is extremely warm where the bottle is mounted, you will never reach optimum bottle pressure (950 psi to 1050 psi). I would recommend, however, if you don't have a good fuel pump to get one before using this kit. Also, anything over the 75 HP setting, will require a good nitrous tune ($400 for a chip). As you can see, the kit isn't the expensive part and by the way, it costs about $45 dollars to fill a 10lb bottle which is good for about 7 or 8 runs at the strip. I use this kit, but I've only used the 75 HP setting.