There are 2 ways, but either way will cost you money.
The standard US server way, buy the dual pack battlechest, WoW
and Burning Crusade pack at 60+ bucks, and the WOTLK separately at
another 60+ bucks. Anticipate Catalysm coming in november, probably
going to release at 90+bucks. If you pay monthly thru credit card,
it'll cost you S$20-22 bucks regardless the days you got play or
not. If you pay for 3-6 months in advance they charge you cheaper
monthly rate, but you have to pay that 3-6 months one shot. Take
note tuesday nights are maintenance days, so no play, and even
after you finished installing WOTLK, there is still almost 5gb of
patches you need to download thru the patcher before u get up and
running to the game. So if you bought all 3 packs up to WOTLK on
tuesday, don't expect to play till wednesday after finishing
patching.
The taiwan server way, I am not so well versed but my colleagues
are playing their WoW that way. Basically, they got the
JustWoW client up to WOTLK from somewhere free. But game time still
need to pay, using game time cards bought from some local website
they showed me. If I remembered its 50+ bucks for 3 months, and
depends on stock. The interface is chinese, but that is easily
overcomed by using a font pack again from somewhere i dunno. I was
told that since I have the existing US server version, I just
needed to mod some values to switch to taiwan server, and buy game
time to start playing, but I have not got into it since I am busy
with BFBC2 at the moment.
The reason for them to play on the taiwan server is mainly
because people on these servers are more objective and into the
game to level up and gear up asap. And also the fact you don't give
blizzard so much money everytime they come up with an expansion. If
you want more information on playing on taiwan servers, go google
for it, I'm not really keen to switch at the moment.
Btw, I pay using credit card monthly, because I want the option
to suspend my account as required. Also I don't really know where
to buy the game cards.