hmm well how much would it cost to run a 24w cfl bulb for 1 year, 10 hours a day, 7 days a week? Do the math!
10 x 24w = 240 watts/day x 7 days = 1680 watts/week x 52 weeks = 87,360 watts/year or 87.36 Kw
I don't know how much electricity is for you but I will estimate 15 cents per Kw
87.36 Kw x .15 cents = $13.10 to run the bulb 10 hours a day every day for one year
You can adjust the numbers for different sized bulbs, different length of time running the bulb each day, different cost for electricity etc.
Do you need to run the bulb all day?? I don't know. my East window gets plenty of sun and a plant in the window gets enough light to grow reasonably well. But if your window is shaded from outside or the plant isn't close to the window then odds are you will need to use artificial light. Bright indirect light in a room isn't going to be enough I don't think.. not without at least some direct sun through the window.