How much does it cost to operate a 1000 watt mercury vapor light?

The same as it costs to operate any 1000 watt device or combination thereof. Your electrical bill is rated in kwh or kilowatt hours. (A kilowatt happens to be 1000 watts per hour). So if your local rate is say 8.5 cents per kwh ( a typical amount ) then it will cost 8.5 cents per hour ( plus tax & service fees, etc.!) Check your electric bill and find out your exact per kilowatt/hour rate. Since this type fixture is a commercial fixture, your rate may be a higher commercial rate than most residences.