From what I've seen for gold coffers that yeild 210 gear depending on difficulty level are:
Easy Mode: <5% gold coffers
Normal: ~10%
Hard: 15-20%

These are the averages I've seen in each mode based on the final tally given once you leave the duty. All 3 were done with consistent pulling and at least 2 groups killing the same mobs at a time to generate tracks faster.