All right, it turns out that the info in my previous post about merit decay was wrong and the reality is a lot more motivating.
The situation isn't that the merits you earn each week only persist for 4 weeks - instead, it calculates your new total using the totals from the last 4 weeks, cumulatively. So, instead of needing to farm 5000 to maintain rank 5 and breaking even with the previously-posted logic, it's actually only 1250 per week and you make 37.5 million profit each week. The 3-4 trade runs needed to get those 1250 merits would be finished in less than an hour each week. Crazy.
This thread goes into a lot more detail.
http://elitepve.com/viewtopic.php?t=2304
"Rank 2: Easy to achieve (just 100 merits!) and easy to maintain: just 30 merits a week maintains you at rank 2.
Rank 3: Can be maintained with 100 merits/week.
Rank 4: Maintained with 200 merits/week.
Rank 5: Maintained, as said above, with 1250 merits/week."