Okay.. Figured a few things out over the past few days.
- The problem with the prices not changing was due to the fact that the sigils were not on the town monoliths. We were in the process of corrupting them again and apparently prices cannot be changed during this period.
- After the sigils were moved back to the towns, I lowered prices and they changed immediately.
- The next day I raised prices again and they, again, changed immediately.
- I tried to immediately set them lower again and got a message stating something along the lines of "you must wait several hours to reset prices again".
I didn't get a chance to try again "several" hours later, but I will be testing that at some point to try and pinpoint the actual timer. Apparently there have been some changes to these timers recently, because over the past year, at least on Siege, the prices did not reset immediately. It used to take 24 hours.