For those of us who are not season ticket holders and won't get lucky in the lottery:
http://www.stubhub.com/washington-nationals-tickets
NLDS 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.
Updating:
NLDSGame 1 - 4,042 left starting at $64
Game 2 - 4,305 left starting at $63.89
Game 3 - 3,876 left starting at $64
NLCSGame 1 - 2,013 left starting at $115
Game 2 - 1,833 left starting at $100
Game 3 - 1,989 left starting at $125
Game 4 - 1,695 left starting at $149
World SeriesGame 1 - 1,761 left starting at $472
Game 2 - 1,670 left starting at $375
Game 3 - 1,673 left starting at $450
Game 4 - 1,669 left starting at $500