Considering Susano'o Sasuke has been in loads of events as well as 4th Raikage and with how outdated Kage Treasure is (for the most part), it doesn't make sense for him to cost as much as he does.

It really does, Kage Treasure has been around since 2.0 at iirc. Everyone has played since then at least has one or the other if not both. Then add in the fact that they've been in events quite a bit even more people have him. Then add in the fact that they are not that great atm (4th Raikage might be an exception), there is 0 reason for them to be as expensive as they are. Now if they had their skillbooks sure, give them a bigger price. But as of right now, they are not worth their price. If the price was determined how rare they'd be, they wouldn't be in any events or be in as few as possible, but that isn't the case.