One of the major determining factors, in answering that question, is what is the ratio of support staff to lawyers in the firm? If we assume for the sake of argument that it's 1:1, which could either amount to a personal assistant for each lawyer or else a shared assistant and some staff that serve the office generally, then a big part of the overhead attributed to supporting that lawyer is paying the other staff. Then the whole rest of the firm's overhead, attributed to that lawyer, is the difference between the lawyer's pay and the staffer's.
Salary is a big part of overhead, but lawyers aren't the only people at a firm getting paid. They are just the only one's who imagine their billings are "theirs" in some rightful way. It always amuses me how often lawyers get caught up in worrying about whether they are being properly paid, to the point that they forget to account for how other people are paid.