Lettuces don't need a lot of light to grow. I recently read "The Winter Harvest Handbook" by Elliot Coleman and in it he describes growing lettuces in the winter in Maine (Zone 5) in the dead of winter underneath two layers of insulating row cover. They don't grow fast as in summer, but they do grow.
The nice thing about leaf crops (which lettuce is) as opposed to fruit crops (like tomatoes that have to flower to produce the part you want) is that as long as you can keep it alive it doesn't really matter how big or vigorous it grows, you can just grow more of them.
I have metal halide lights indoors and the lettuces that I plant underneath them flower and go bitter in about 8 weeks - hardly worth the effort.
My advice would be to go to home depot and get some shop lights which are 40W fluorescents or something similar. They are cheap and easy. You can spend money for the grow bulbs but I would not bother at this point. Start small and experiment, you can add more if you like.
BTW, if you are in a cold climate where the inside air in the winter is very dry you will get better results if you enclose your growing area as much as you can and do everything you can to keep the humidity up.