Assuming you want to see (and not just be seen), I have a 650 lumen NiteRider light that works pretty well to see where I am going. Cost around $40. If I had to do again, I would go for a 800-900 lumen light, primarily because it has a bigger battery (rechargeable), so I could get longer run time on medium setting.

To simply be seen by cars and not to use the light to see the road, a 100-200 lumen el-cheapo light is fine for that.