There's been a bit of blog chatter lately about a special Alaska Airlines is running - offering a 5,000 miles bonus - to join the Alaska Board Room (their airport lounge club). The basic membership costs $450 for a year, though could be as low as $295 if you have high status on the carrier.
American, Delta, United and US Air all have about the same rate card for joining their clubs.
I scratch my head as to why anyone would pay this sort of money to join a single lounge. ...

