There are many things to factor in, but generally speaking, you could expect about 1 lb per light every 4-5 months. 1/2 lb per plant.
As for monthly overhead costs in Southern California, you will spend about $5700 per month total for electricity and water to run 50 lights with everything included like water/air pumps, scrubbers, wall fans, dehumidifiers, Co2, and general lighting to walk around.
Factors that determine the yield.
-Vegetative time
-Lighting Layout
-Type of Hydroponic System used
-Strain
-Experience of grower
It's all about planning, but the great thing about growing is that you can plan and forecast fairly accurately. The factors I mention above will make a huge annual difference in yield per year.