play asia is a great website to buy hong kong $ psn cards.
$200 hong kong is about £17.87 GBP at today's exchange rate (oanda.com fx currency converter)
Play asia will charge you £20.92 GBP, so you will pay about £3 GBP extra for the priviledge, or about 12%..
It really depends how much you reall want that game before everyone else..
I am glad I bought Everybody's Putter Golf, as it was about £4 and apparently will never come out outside Hong Kong, so it's a bit special. It also has a good trophy list for such a cheap game.
hope this helps