The cheapest ready made bicycle light I can recommend for you is going to be the Planet Bike Blaze 1 watt at $40.
http://www.rei.com/product/769616
It will allow you to ride fairly safely under good conditions as long as you don't go fast. Others try to get by on less to see with, but they are running more on luck than light. Under some conditions though, even lights brighter than the above will not be enough. Freshly laid black pavement after a shower will make it near invisible. Older faded pavement that is dry will make the light seem several times brighter.
If traffic is only a very minor concern you might get by with a fairly cheap tail light ($10), so long as you aim it properly. Keep your reflectors on too though. A couple bucks more can get you a very decent light though.