There are only a few two family houses in Park Slope (above 17th St) currently on the market, and their asking prices range from $1.3 million for a wood-frame house on Prospect Avenue, to $1.5 million or so for a nice, under 20' wide house in the north Slope, to over $3 million for an extra-wide landmarked mansion in prime center Slope.
Taxes vary depending on the lot size, square footage, and the number of times the property had changed hands -- in the above examples, taxes range from $2K to $7K to per year (and it isn't the most expensive house that has the highest tax bill).
Your best bet is to work with a real estate broker who is completely "plugged in" to this very small market. Many high-end properties are never openly advertised for sale, due to their owners' desire for privacy. Please let me kow if you would like any further information on any of these listings.
Happy New Year and good luck with your search.