The more watts the bigger and better your yield will be I would go with a 400 hps... As long as you have good ventilation, those bigger lights tend to get pretty heated. As for month to month cost, I take it that you mean electricity? If your vegging then flowering at different times I wouldn't expect it to cost you anymore than 25$ extra a month for everything... And thats for your 18/6 cycle. Cheaper for your 12/12.. Its fairly easy to ballpark the price of what youll be spending. All you need to know is how much your electric company charges per kilowatt hour... (KwPh)... Then from there its simple.
You take whatever wattage light youll be using. Say we use a 400 watt. Ok, So you take 400 divide it by 1000 to get (.40 KwPh). Take that .40 and multiply it by whatever your electric company charges KwPh around here its 8 cents. so (.40 multilply by .08=.03) So thats 3 cents to run a 400 watt system 1 hour. So you take 3 cents and multiply it by 18 since we'll be running the system 18 hours a day and you get 54 cents. So thats 54 cents a day. Since theres 30 days a month (on average) you take 54 cents multiply by 30 and you get $16.20. So to run a 400 watt system 18 hours a day for one month your looking at around 16bucks extra. Not a whole lot at all. And its even cheaper when you start to flower at around 11 bucks a month. So hope this helps you out some...