As for volumes or smaller sets, they tend not to sell so well. How many people would prefer to buy parts of NICOT/NICNT rather than the entire set?
For me, economics is the primary driver of my purchasing. If I had the money then I'd spend about $2K or more right now on commentary sets and other things. But in the real world, I can't do that. So, I try to figure 1) how much can I spend and 2) what way of doing things would be the best value. For example, I've been thinking about buying the NAC commentaries. Right now I can't afford the full price for the entire set (even though it's on sale). However, I have resisted buying any of the smaller sets of this series because the value is not as good as buying the whole set all at once. I would probably be more likely to buy the set in pieces if the total of all the pieces equaled the total of the whole set. I understand the advantages of the marketing scheme for not doing it this way, but the bottom line is, you have one customer who has decided not to spend any money on the commentary series because of it. The situation would be the same with NICOT/NICNT. I imagine that the cost for the entire set would be out of my reach for a while and would probably want to buy individual volumes as the need arises (or would buy smaller bundles wherein there is only one volume I need at the time). But even with this I would hesitate if purchasing individual volumes or small groups ended up with me spending several hundred dollars more than the price for the entire set.
I'm sure not all are in my situation, but I'd bet there are many that are (particularly students). I think the smaller groups of volumes would sell better if the value on them would collectively add up to the value of the entire set. Does that make sense?