Correct, thought the price could be highly variable. I have seen those vary by as much as a factor of 4 on the same route and class of service.

I understand why AC does not release inventory to UA on flights that are in high demand. But I am a bit at a loss to understand why they don't release inventory on low-demand flights. For instance, when AC charges 22,000 Aeroplan miles for a flight from DFW to YUL in business class, to me this signals that this is not a route/flight in high demand. Why would they not also release inventory to UA in that case?

BTW, I just purchased some Aeroplan miles with 85% bonus and completed my transaction on the AC side. I am very disappointed, however, that I could not use my UA miles for this.