Hi Janie,
This can get to be a real complicated issue. It varies for just growing transplants versus actually "growing plants indoors". There are a lot of good posts on this subject if you search for "shop light grow lights" you'll hit a number. The wattage is pretty much determined by the type and length of tube.
Flourescent lights have changed a bit in the past several years. The regular old four foot T12 fixtures and bulbs are still around, the least up front expense and a lot of people have grown a lot of great transplants just that way. Four foot units are 40 watts per bulb. You can get a cheap T12 shoplight at Lowes for about $10 and two Daylight spectrum bulbs for about $8. (Plenty of folks use regular cool white but I chose Daylight as I also plan to culture some aquatic plants for my aquariums.)
The newer T8 and T5 units are a better choice (unless up front money is really tight) with better power efficiency. That same four foot unit in a T8 will use 32 watts and actually put out more light than the 40 watt T12s. T5's can tend to get pricey and I only use them over aquariums.
Just this evening I set up two four foot T8 units from Lowes with GE daylight bulbs (2700 lumens at 6500 Kelvin color range). Cost to get two fixtures with , 4 bulbs (covers four flats of plants) was about $54 including tax. I addded another $8 for some 2x2s and bolts with wing nuts and washers to fashion my frame for suspending them. Total for my whole setup was right at $62.
The T8 setup cost about $16 more total than it would have cost to use T12s but I think the T8s are well worth it. I'll try to get a picture and post it tomorrow evening.