Less than $20.00 a month
Peace
I'm in New England as well...my 400 watt HPS + (2) 42 watt CFL's boosted my bill by about 70 bucks a month....apparently your mileage can vary.
There is so much you can do to lower your electric bill or offset it.
Wash laundry in cold water only
Lower thermostat on water heater to 120 degrees
Put timer (called gray box) on water heater huge $$$ saver
Line dry laundry
Use motion sensors outside instead of lights that stay on all the time
Depends on your electric company, there rates and associated taxes and fees.
Peace
(Edit) The more I think about your post, something has got to be wrong here dude. $70.00 for one hps and a couple of cfl's? No way. There's more to this equation that just those lights you mentioned.
The one thing i didn't mention was my vortex fan which is another ~ 80 watts. Add it all up...for a 30 day month thats an additional 304 kW hour...19.1 cent per....thats ~60 bucks. How much are you guys paying per kW hour?
mine is 8 cents man are you getting screwed not even NY City is that high do google on it