Looks like, according to the buddy fare calculator, it would be 124.81 to go down, and 87.50 to get back (taxes are probably different). I don't know how closely that matches the real buddy pass price, but from my experience it's probably pretty close. As for your likelihood of getting on, it's impossible to say even when it's the next day, so trying to check now for December would be pretty useless. If you've absolutely got to be there, the you should buy the confirmed ticket. $400 seems steep, though — check travelzoo.com and fly.com and see if there are better deals. You have a lot of time ahead of you, too, so you probably will get better deals before then. Finally, I know some online sites, like expedia, offer fare alerts, and they send you emails if the fare drops below a point you specify.