@Dan P.
Yes, that example is pretty normal. Listings agents are basically highly paid used car salesman. Having said that, most of them are very good people but understand they are trying to get their client and themselves top dollar.
You are in a different and much more sophisticated arena with MF so it is up to you to uncover what the more appropriate numbers are. With experience you are able to build out a complete and detailed expense proforma based on your knowledge of what an asset will take to run on everything from staffing, marketing, admin, legal, utilities, etc...
Your approach of 50% Op Exp is usually close enough, if it's not an ABP property, for back of napkin numbers to see if the deal is worth digging deeper on. Then you have to verify their proforma rents and even if they are relatively accurate in the current market, will that market hold out for you as you try to drive the rents higher?
Always be extremely careful when making an offer based on ANY proforma in a hot market, you can get burned if it corrects on you.