I recently started leveling my rogue again, partially for fun and partially because I want to expand into the leatherworking market. So while dusting her off and cleaning out the bags and banks, I found a couple of Mammoth Mining Bags that I had tried to sell at the beginning of Cata without much luck. So I tossed them in the AH, undercutting some guy who was posting at about 300g. And they sold. So I made a couple more and put them up. Those sold too. I started back on my rogue a little less than a week ago, and so far I have sold a total of 12 bags for an average of ~320g each, and they cost me probably less than 80g to make!
So how do you start making your own? Well, if you don't have the pattern already, you'll need to be honored with the Son's of Hodir first. Fair warning, the initial quest chain is a bit long, but you should be honored fairly quickly after the initial stages. The bags cost eight Heavy Borean Leather, which you can make with 48 Borean Leather or just buy off the AH. And that's all there is to it!