Good guess. I doubt the price can fall much lower without venturing into "loss leader" status.
We can also add that Best Buy has dropped out of the price reduction entirely. They've evidently sold off all of their unlocked N6s and are now just trying to sell off what's left of their carrier-specific units. T-Mobile has also eliminated the Nexus 6 as a "new" item, leaving only pre-owned versions available on their web site, and I imagine other carriers are following suit.
Besides, now that the 5X and 6P are out, Google's not going to want to keep pushing the N6 as a bargain. They made a strong pitch for both the 5X and the 6P. They want to see that new product move. (And it looks to be mighty good product, too.)