Singles are at a higher price because people who just need a handful for a synth are willing to pay to get the exact amount they need. Why pay 40k for the 6 you need and 6 that are gonna junk up your inv and AH slots when you could pay 24k for exactly what you want?
The people who are selling stacks are most likely crafters who are skilling up on the synth and have a lot they want to unload quickly to free up inv space to skill up more. So they discount the merchandise to get it to move faster.
