I don't know about the price going down. At least not for a while. The gobbiebag market targets just about every player. The gobbiebag item is HQ3, which isn't all that common.
The price was 4k gil when I started selling these. It has gone down, I had to take one back from the Jeuno AH because it did not sell at a 2k price. I'm going to garden for my primary craft. I am beginning to think Wild Onions are better than Black Rocks