pity that you bought them in the same week, otherwise we could find out who contributes what. It is obvious, you must sell one of them and then tell us :-)
merchandise is more long term, so he won't get a single race in the next week but he get several races over the next 4-5 weeks in income. So the problem isn't the same week, maybe more the same month ;)
So long term is between one to two weeks according to the example you provided and my experience with NT nominations. No that long term I guess and this doesn't also mean that somedoby gets several races but just one.
Another theory in merchandising dictates the critical factor is that players should get minutes in order to recieve most of the income for merch.
but also the minute thing is a calculation of the last few weeks, i didn't notice a merch drop when one of my stars was injured for two weeks or when i play scrubs in the league. Ok it isn't a constant factor, but it is normally in a +/- 2% range.
Maybe citb could give you values for his increase to, because he posted the same increasing theory when yoginger complained first about his merchandise in our league forum(right after the drop).
PS: i expect at least another 20k increase for yoginger in the next week, because his roster should get in the same range like mine.