With most, though not all, overseas domestic helpers are expected to work 6 days a week with one day off, some even agree to work 7 days a week. The cost of 6 days a week on a monthly basis is a minimum of NT$ 15,840, ie minimum wage as agreed between the countries concerned. This is also approx - leaving aside exchange rate fluctuations - what they earn in HK and Singapore. If they work the extra day then the cost rises by NT$ 528 for each extra day worked.
Depending on the role of the domestic helper then most will also help around the house, in terms of cleaning, cooking, washing, ironing etc, something almost without fail that a local nanny will not do.
Total cost per month for overseas assuming no overtime and employed based on salary is just under NT$ 27,000 per month. On top they are entitled to 7 days paid leave per year, most take it as extra cash at the end of the contract. Annual cost under NT$ 330,000 per year.
When we did the calculations some 5 years ago, the cost of the local nanny per month was about NT$ 20,000, but this was for shorther hours, from memory i think 8am until 7pm and only 5 days a week, and we were expected to take the child to their house/property. In addition, there was bonuses that had to be paid during the year for Dragon boat, CNY etc. Total cost without overtime, which for us would have been a bit was NT$ 280,000.
For us, the difference of NT$ 50,000 would easily have been eaten away with overtime and weekend working, so we went the maid route, but it takes a while to get everything sorted first time round, in total about 3 to 4 months, and that assumes the maid you choose has not problems with visas etc. The other factor for us, was that she would be live in, so no need for the child to be taken away from its own environment.