for wow, i pay monthly via credit card and it costs me 24.91 per
30days.
it is cheaper to pay via game card (you save ard $2-3) but if your
time runs out, you can't continue playing unless u got a spare card
with u all the time.
recently i found a source that could get me wow game cards at $40
for 60days so i'll be switching to that soon.
as for the game, yes the prices for WoW has dropped drastically.
main reason being the expension pack - burning crusade (aka BC)
will be released soon (16Jan is the tentative date). I also found a
cheaper source for BC so if you are gonna play, do let me know, TX
and I would be glad to help you get started 
