The siege has been on for almost a week now. There is still another week to go and most of the prizes are still up for grabs.
Of particular importance to the guild however are the stronghold vouchers. They drop from killing any monsters in the siege event area. It appears that they drop approximately 1x per minute for regular monsters and 3x per minute for some heroic encounters.
Its not necessary to end all the HEs immediately. Hang out and kill the extra adds that are made with the end HE boss comes out. Quite a few extra vouchers can be produced that way. It is however hard to find an instance with few enough players in it to get this done very often. Most often the boss gets mobbed and killed in seconds, which is a bit of a waste.
Between 30-60 vouchers per hour can be grabbed out of this event. Per time spent, this is a far more efficient operation than any other for getting coffer value. You even get influence vouchers, and over the course of an hour you'll get more than your daily 400 that would have been received from stonghold HE runs.
'Dragon hoard coffers' also drop in this event, and a little faster in the siege area than they do in the Well of Dragons. In WoD its approximately 1x per 10 minutes, while in the siege its about 1x per 6 minutes. Btw, these are useful for getting the last 2 boons in the Tyranny of Dragons campaign, and it takes Oh so many of them. (8000 coffers total). So soak these up while you can as well!