Trial of Style will have a specific outfit slot that will allow for free transmogs for that outfit slot only, and is removed once the event ends. So no more free transmog for anyone.
Currently all transmog prices are priced for max level characters regardless of the character's actual level. Don't know if this will be fixed or not, but every now and then some of my characters get prices appropriate for their level and I fill up my outfit slots for cheap. No clue how or why the transmog prices become pre-patch prices, but if for some reason you see cheap transmog prices take that opportunity to just transmog all of your outfit slots.