It sounds like your camera-mounted video light will be your "primary" light for the night dives. That will give you very broad illumination which is great for night diving as you want to see as much area as possible. If so you are really looking for a back-up light.
If you have a broad video light already, I would suggest a back-up with a narrower beam. That makes it useful for signalling your buddy. In your price range, I think the best bet is the Big Blue 250 lumen focusable light. This gives the best of all worlds. Decent brightness, and a focusable beam from 6 degrees (very tight for signalling) to 30 degrees (great for general night diving). Price is about $150 with the hand-wrist mount. Light is very small and uses standard AAA batteries.
The novae SST-50 is a VERY bright light for the money ($130-150 or so). I have 2. They are standard flashlight size and so too big for a hand holster. When they work, they are absolutely superior and give more light than units costing $800.00. Way, way brighter than the Big Blue. They have a narrow bright central spot and a very bright and wide periferal area. I would consider them the best night dive light I have used, HOWEVER there seem to be reliability issues with the units. The re-seller, Novae, is absolutely great for service and standing behind the product. However, the lights themselves seem to be uneven in their quality and reliability, as do the proprietary rechargeable batteries. If you get one, you also MUST get a SECOND back-up light to carry with you, and factor this into the cost. In other words, would not dive with just your camera light and the SST-50, you would need a third back-up as well.
If you can afford it, get the SST 50, and the Big Blue ($300 total) and you will have a really good combination.
If you just want one light, I think the Princeton Tec Shockwave LED, or the UK Light Canon LED are the best choices in your price range. They claim about 800 lumen brightness, which is a lot. They are $200.00 or less and use standard "C" batteries. They have super reputations for reliability and bullet-proof construction. They also get plenty of burn time for several dives. They have pistol or lantern handle grips. Check them out at your local LDS