The cost is not in the millions (Maybe over a period of many years)
You dont need a Brownstone in Brooklyn, you can rent a non-decript office in a non-chassidic part of Brooklyn for about $10,000 a month (Privacy needs to be important so a non-jewish area would be better)
You need 3 or 4 staff members to start, A secretary, Social Worker and a Rabbinical Advisor and a place you can rent (If if the office is not big enough) for group events once a week or one a month.
You can do for about $400,000 a year and you can combine the Social Worker and Secretary at least at first to save some funds. You also might be able to get some rooms at some shuls for free or discounted also to save some funds