Natural sunlight is as follows:
120,000 lux Brightest sunlight 110,000 lux Bright sunlight 20,000 lux Shade illuminated by entire clear blue sky, midday 10,000 - 25,000 lux Typical overcast day, middayGenerally with starting any plant inside earlier than you intend to put them outside garden centers will recommend a couple CFLs saying that at this time of the year the light from a window wont be enough and will cause stretching. As I said before, you can get a 6 pack of 23 watt CFLs for like $15-$20. Home Depot carries up to 100w bulbs that are inexpensive as well (the wattage I refer to is the actual bulb amount and not the incandescent equivalent). That is probably the cheapest start-up costs you can get.
You should check out this thread:
Things to know about lighting