I've never stayed in one before but from what I've seen you might be able to get away with 8,500円 a night since you want to share a room. Don't know if it's any good or not but I just randomly googled some Ryokan's for example prices and this was about the prices I got.
NISHIYAMA RYOKAN Reservations
As I said though I've never stayed in one but it looks like you might find somewhere.
Also, I see your avatar is relevant to the topic
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