Estimating realistic operating/capital expenses ?

What percentage (%) of gross operating income (GOI) should one expect for annual operating & capital expenses ?

Is 50% a reasonable starting point?
or does the percentage vary by the property class ?
e.g., Class A - 30%, Class B - 40%, Class C - 50%, etc.

OR instead of %, do investors use $/unit, e.g., $4,500/unit annual expenses ?

Much appreciate your help.