Dead Men Working has a nice response to the GLIFAA letter.
Arguments based on cost would certainly not stand up to scrutiny. Expanding existing benefits would cost money, of course, but that money would be buying a happier, more diverse, more effective and more productive workforce.
The Department pays widely disparate costs to transfer, house and accommodate FSO families, and modifies those costs to accommodate a variety of personal decisions.
For example, in an age of birth control, having children is a personal decision. An FSO who adopts a child, or who decides to have another child, or who takes custody of a child from a former marriage, can expect for that child to be transported, housed and educated at government expense. There is no rhyme or reason to this. Nobody limits the number of children a Foreign Service Officer can have or adopt, and there are some who have eight children or more. So one traditional family might cause the Department to support ten people, another only two, both related to employees of equal rank and cone; and nobody mutters much about the difference.
Fairness is over-rated.
So, Madam Secretary, don't do it for fairness.
Do it because it makes good business sense, improves our efficiency, makes the Foreign Service more representative of American Society, and makes it easier for the Foreign Service to perform the tasks we were hired to perform.
I recommend you read the entire piece, which, as always, is well written and argued. I have personally made the budgetary argument to the Under Secretary for Management. I pointed out that budgetary arguments really can't be considered because, aside from the miniscule amount of difference it would make in the overall budget, the Department can no more budget for an office remaining single (at least for budget purposes) for an entire career any more than it can limit the number of children an officer has.