Unfortunately paid Android apps are currently unavailable in China & Taiwan amongst others (for a full list of paid-app countries see here although it’s not always up-to-date).
Google Play Store determines your “country” by checking which country your sim card comes from. If it comes from a paid-app supported country then you will be able to view, buy and download paid apps. Otherwise, they will be hidden away from you.
So, the simplest way to buy paid apps is to temporarily swap out your Chinese/Taiwanese sim card for another sim card from a paid-app country. It does NOT need to be active (you can connect to the Market using WiFi). With your US or UK or German sim card in place, you should be able to buy paid apps. Once you have done so, you can swap back in your Chinese/Taiwanese sim card and happily use your new paid app trouble-free.
If you do not have a paid-app-country sim card then unfortunately things are more complex. Please read about MarketEnabler.
More information can be found here.