Is 관리비 negotiable, or is that a fixed thing?

Negotiable? I doubt it, usually landlord just says one number and that's it. But asking what is inside 관리비 is not odd at all, nobody should sign for 별도 with nothing written

"별도" is doing a lot of heavy lifting there

lol a postdoc asking for an itemised breakdown before signing is the most researcher thing ever, next you're gonna want a methods section and error bars on the stairwell cleaning ㅋㅋ ngl tho 관리비 is lowkey the scariest word in Korean, way worse than when 원장님 says "quick meeting"