My gut feeling is that one day the dreads may spawn again, so if you're thinking about buying one, I would only pay what you're willing to shrug off if they respawn later. Don't pay out if you'll regret it later

I'd wait at least until the end of this event if you can, because that's logically when any planned respawning will occur.
You shouldn't get guardwhacked by saying trick to a dread in town, as it doesn't do anything more than a display effect.
Wenchy