I always treat arranging specifics with a sugarbabe as a proper negotiation. I don't date professionals with fixed rates, so the amounts depend on chemistry, attractiveness, expectations, etc.
I've had candidates regale me with stories of men lavishing millions of yen on them in exchange for nothing more than dinner company, but when my reply is "cool! I'm sorry but I won't offer anything like that" they're quick enough to discuss more reasonable arrangements. So I don't take initial offers very seriously, except as an immediate "no" when they're totally unreasonable which is a red flag for me.
All that aside I'd say 30-50,000¥ per date is reasonable to start with for a girl in the 6-8 range in terms of looks. If chemistry is good a monthly arrangement could be anywhere from 2-10x that depending on conditions. Of course if she's young and closer to the 8-10 range it could go a lot higher