For those of us who are not season ticket holders and won't get lucky in the lottery:
http://www.stubhub.com/washington-nationals-ticketsNLDS tickets can be had from $60 up
NLCS tickets can be had from $100 up
World Series tickets can be had from $300-500 up
About 3,000 tickets to each playoff game are already being listed on the secondary market. I imagine that the supply will substantially increase after the initial sale to the public.