It's good timing for your question, Aaron, as I have checked this out recently. The reason I am into this is to find the lightest way to carry the canister with FAST and SECURE access. In 2006-07 I left the stock holster in the truck and just carried the canister in a back pocket that held it perfectly. But I had to remove it when I took a short break and sat on a log. In 2008-10 I didn't go to GNP or YNP, so I didn't carry bear spray. In 2011, I simply attached the canister to the sternum strap, like this:

One day I came face-to-face with a huge grizzly, and I had to quickly access the spray. The female part of my Osprey Hornet was very short, and I had a heck of a time getting the sternum strap released. The longer female part of the strap in the above photo would probably allow fast access. But it still takes two hands to do it. I needed a better approach to carrying the canister.
So I came up with a velcro strap idea to securely attach the canister to the shoulder strap. It still took two hands–one to release the velcro and the other to grab the canister. But it is pretty quick, and very light (4 grams).

I wanted to find a way to grab the canister with just my right hand, to speed things up and be easy. I recently bought a shoulder strap pocket from Gossamer Gear, which works well and can be used for other items when not carrying bear spray. But it is nearly as heavy as the stock holster at .9 oz. It isn't very clear in the photo, with the pocket and the shoulder straps both being black.

I pulled out the cuben and designed my own canister sleeve. It has a belt loop next to the shoulder strap which I run a 1/2" strip of velcro through to secure the holster to the shoulder strap. The opening at the sleeve top has a shock cord that can be adjusted with a tiny cord lock. This allows me to tighten it just right so that it won't fall out, but the cord will stretch when I pull out the canister. Then, at the bottom of the cuben sleeve I made a cuben loop. I inserted a small Scunci hair tie into that loop and secured it to the tri-glide where you adjust the shoulder strap. This stabilizes the sleeve so that it doesn't bounce around when hiking. This cute little cuben sleeve and velcro weighs 0.4 oz., and I think it looks pretty cool.

But really, there is a very simple solution for quick one-hand operation that weighs a scant 3 grams–2 Scunci hair ties! This actually works. Some of you already know that I'm a huge fan of silicone Scunci hair ties, the best rubber bands on the planet. I always carry 5-6 extra with me, so I always have a spare in case one breaks.

Now back to your question about weights:
My original Counter Assault holster weighs 32 grams (1.12 oz.) and my full, but expired canisters weigh between 281-312 grams (9.9-11.0 oz.). I have wondered if there might be an imperceptible loss of the contents over 5-6 years, or if there is just a variation in the amount of the contents.
Last week, I went to REI and I weighed all of their 8.1 oz. canisters (there were only 6 of them on the shelf). they pretty much all weighed the same (331-333 grams, with an average of 332.5 grams, or 11.73 oz., in the original packaging). Their current holster has been put on a diet, and it now weighs an average of 25.56 grams with the minimal packaging (sample of 9 holsters). So the current holster is a bit lighter than the older, original version.
I will be presenting this information at our March Boulder Lightpackers meeting, so Gordon and Stuart and the Randys can take a coffee break when I talk about this stuff.