機場 upgrape to Business Class 要如幾多錢?

Two things:
1. The price is not fixed.
2. There must be seats available in Biz Class so that they can sell it to you.

I once managed to get an upgrade (from HKG to LHR) for HKD$10,000 in July.
It can cost you more, or less, than what I paid. A matter of supply and demand,
as well as luck ... so good luck to you.

PS. Ask the status of your plane (whether it's full or half-full) at check-in, then
act accordingly. Always be polite to those who help u check-in coz they're
really in control of where to put u.

U might get an upgrade without even paying a cent if the check-in guy likes
you (from my own experience). Anyway, there's no harm to try/ask for a free
upgrade first ...