Just go and camp the chest and get it. The drop is a little more frequent then you would think. It's also a good way to make gil, as you have to farm the key, unless you're a THF and then all you need to do is pick the lock. But I do suggest doing this is saves the gil for the stuff thats harder to get.
I realise that this is about 2 years out of date, but.......
if hardly anyone on your server hunts coffers, why are you wasting your time trying to buy it off AH?
Finding coffers on Asura is difficult because alot of players will actually take the time to hunt them down, you however have the perfect opportunity to expoloit this lack of interest on your server and save you some gil in the process