OK, am slightly puzzled by the outcome of my quick test mining run, because firstly the "1 at a time" route is apparently yielding fewer merits than "all at once" for me, and more interestingly the return rates weren't behaving consistently for different commodities so maybe something else is going on.
I came back with 31 Tritium, 1 Bromellite, and 14 LTDs. This was in a Fortified system btw.
I sold 1 Tritium first, getting 40 merits for 57585 CR, thus 1439.6 CR/merit, and then the other 30 together, getting 1296 merits for 1727550 CR, i.e 1333.0 CR/merit - i.e. a significantly better rate (by massively more than the round-off error associated with the 40 merits).
The single Bromellite went for 72767 CR and gave 52 merits - so 1399.4 CR/merit there. This is also not consistent with it being the same CR/merit return rate barring round-off error: even if I had received an extra merit (53) that would still have been a worse rate than for the batch of Tritium, at 1373.0 CR/merit. So, is there some kind of offset or threshold, or a commodity-dependent ratio...?
Finally, the first LTD sold for 201106 CR and gave 148 merits, thus 1358.9 CR/merit, with the final 13 going for 2614378 and giving 1964 merits, thus 1331.1 CR/merit. This is for sure very nearly the same rate as for the batch of Tritium, but yet I still can't find a single "master" CR/merit ratio which would round up/down/nearest to the precise number of merits I actually got for the Tritium and LTD batch sales. I guess I have to interpret this as meaning that something else is going on, but it's early days and some of you may already understand this stuff well enough to fill in the blanks.