I would stay away from the blackburn quadrant, or any other be-seen light. The Quadrant is great for drawing attention to yourself, but useless for actually seeing the road at any speed greater than about 10mph. In the conditions we see here in CT, the back roads are dark enough, and the traffic just heavy enough, that unless you have a light that is bright enough to prevent you from losing your night vision when dealing with an oncoming car's headlights, you'll basically be blind.

So, for under $100, what can you get that is bright enough to see the road? There are basically two options. One is to get a halogen light(at minimum 15W), which will give you a lot of light. However, you will need a large battery(big enough to take up a bottle cage), with all the weight that entails. I'm not fond of the color of halogen light--IMHO, it makes the road look like sand. However, if you want a lot of light, cheap, and don't care about weight, it's the best choice.

The second option is to buy a fenix L2D flashlight, a bike mount, four NiMH batteries, and a charger. Compared to the halogen route, it will give off about the same amount of light as a 12w halogen or thereabouts. However, it has a tighter beam. This lets the flashlight throw more light downrange. The disadvantage is that the light is useless for seeing anything off-axis. The benefits of the Fenix route is that even with a backup pair of AAs, is FAR lighter than a halogen + battery, and will give over two hours of battery life on its highest setting.

Personally, and I know this is higher than your budget, I think it is better to buy two fenix lights rather than just one. The first reason is redundancy. If you fail to charge a battery, or something else goes wrong, you always have a second light to get home on. The second reason is that you have more flexibility in aiming the lights. You can aim one light close, and one far, so you can see the road imperfections right in front of you, and any nighttime critters farther away.

In any event, I would not cheap out on a light. You see a cheap light in the store, take it home, then you light them and gaze in astonishment at how "bright" it is, then you hit the road, find your favorite long descent, and realize that "Oh, carp, I'M BLIND!"