To give you an idea on weight, according to my scale it weighs appoximately .3 ounces including the two mounting screws which are included with the Monkey Bar.
IMG_20150525_172141759 by , on Flickr
ETA: The Monkey Bar will fit pretty much any light out there (Executive Series, X300 etc) with the right Fusion accessories but for this part I am just going to mention the Scout light.
For comparison (and strictly doing only a weight comparison of Scout light mounts) the Haley Dropwing Light Scout Mount weighs 1.16 oz according to Brownell’s.
http://www.brownells.com/shooting-accessories/flashlights-accessories/flashlight-mounts/adaptive-light-mount-prod53906.aspx
Noveske’s Keymod Scout mount is .8 oz.
http://shop.shopnoveske.com/collections/parts/products/km-slm
Arisaka’s INLINE SCOUT MOUNT- KEYMOD is .6 oz
http://arisakadefense.com/collections/light-mounts/products/inline-scout-mount-keymod
Weight of MBUS Pro and Monkey Bar with hardware:
IMG_20150525_172154507 by , on Flickr
I know these aren’t exactly apples to apples but its hard to compare the Monkey Bar to anything else out there because of the unique mounting system. For those of you serious ‘ounces equals pounds’ dudes out there the Monkey Bar is by far the lightest light mount of the 3 I randomly chose to compare with. The advantage to using the MBUS Pro as the host mount is that the weight savings and compactness are definitely a plus. Also I like being able to switch the rifle to the support side and grab rail and not a light mount with my support hand.
Some of you have had concerns with the robustness of the Bar if the light takes a good hit. It is made from 6061 aluminum and it is much thicker at the mounting point for the MBUS to help compensate for the lack of a steel clamp. Would love to try this out but I don’t have a spare Scout light I am willing to try and destroy. 
IMG_20150525_173303845 by , on Flickr
Like I said before the Monkey Bar (Like Unity Tactical’s Fusion Hub) will take many of the Fusion accessories. This review featured the Scout but with the Fusion ring light mount (available in .825" and 1") most flashlights will work with the Monkey Bar.
http://www.unitytactical.com/shop/fusion-ring-light-mount-0-825-executive-series/
http://www.unitytactical.com/shop/fusion-ring-light-mount-1/
Also looking forward to trying out the Offset Rail on the Monkey Bar and trying out an X300.
http://www.unitytactical.com/shop/fusion-offset-short/
I apologize for not having pics out with these products mounted on the bar with the appropriate light but I am out of town and do not have access to my gear at home. Will try and post some up next weekend.
This is just a nice simple well made piece of kit. I really like how compact it makes everything up front. Especially if you are running a shorter rail this blending of a very compact front sight with the Monkey Bar makes a good combo. Thanks for your time!
http://www.unitytactical.com/shop/fusion-monkey-bar/
A few more shots for reference:
IMG_20150525_180752775 by , on Flickr
2015-05-25_05-28-56 by , on Flickr
IMG_20150525_172247244 by , on Flickr
2015-05-25_05-24-19 by , on Flickr
-Jax