For me this bag wouldn't work. Seems to hard to get into and I like to be able to stop, get the camera out and take some shots without getting off the bike. I don't know if you plan on using a camera during the day when you're riding, or is the bag is just for transport but if I had to struggle so much every time I want to take a shot, I just wouldn't.
That Topeak bag you posted the link to is no longer available. But as I said before, I use a Topeak bag and love it. It's wearing out after many years and I plan on replacing it with exactly same one. I love my Ortlieb panniers, but the design of Ortlieb handlebar bag is not for me.

I suggest that instead of just looking at the price, try to imagine if this is the bag and set up that will work for you. It's not a saving if you have to replace the item because it doesn't work.