This is a really useful list! Finding out the average prices of certain items could be confusing for a new player, so I can definitely see this coming in handy.
Here's what I'd add:
The price of blank scrolls (I think they go for about 200-300 ryo? I'm not entirely sure)
The price of sealable scrolls (I've seen people selling them a few times, but not often enough to be sure about the price for each tier. Sealable III may be 3k maybe? Don't take my word on this, I've never bought a scroll)
Information whether cash shop items are tradeable and if so, what the rates are (I actually don't know the answer to this question, but even if they aren't, I think this thread could be a good place for information about this) (if this thread is intended to be only about drops, ignore this and the previous point)
The price of drops like the double fan, twin fangs etc. (Double fan might be going for 5k? Again, unsure here. Sellback price is a little above 1k, 1250 maybe. Someone correct me if I'm wrong)