The reason EVA and CAL pay their expat FO's so little is cause they pay their Taiwanese employees even less. This causes problems - an expat FO cannot be seen to earn more than a local Captain - very bad for morale. The only people who are more unhappy than the expats at these companies are the locals - they really get treated like dirt and are tied to the job through bonds and the fact that the Taiwan Licence is not ICAO.
It is only worthwhile joining as a Captain - you can get a fair deal that way. Some Expat Captains get us much as US $ 15 000 or more. Depending on their negotiating skills. Many different salaries and contracts - changes constantly. If you really really need jet time then it might be worthwhile to sacrifice two years or so over there and get some hours that will enable you to move on.