any check in desk or the customer service desk airside. the customer service desk airside is the easiest in my experience.

whenever I ask it's about $300 for wt->wtp and $700 for wtp->cw and $1000 wt->cw. that seems to be the same for either direction to east or west coast us. not sure if Asia would be the same price.

I don't consider it very good value personally. if you find a good cw fare you can get a confirmed business class seat (including connections) for less. the only time out makes sense is when the flight you want is full or nearly full in the class you want but there's a good fare available in the lower class.

the seat map is useless for these purposes though. ba charge for seat selection even in business class (believe it or not) most stays are usually only assigned at check in.

also make sure you get your points and tier points of your collecting avios on ba exec club. ba (uniquely) give you the higher points and tier points but often the computer doesn't do it automatically and you have to call.