There is no minimum wage for local domestic helpers under the laws of Hong Kong.
As for foreign domestic helpers, you should pay them no less than the prevailing Minimum Allowable Wage at the time of signing the contract and factor in the living costs of your foreign domestic helper. The Minimum Allowable Wage has been adjusted to HK$3,480 per month with effect from 6 June 2007.
In addition, you have to pay an Employees Retraining Levy which will be used for the training and retraining of the local workforce and related promotional activities to enhance their employment opportunities at HK$400 per month for the contractual period, either in one lump sum or four equal instalments. More details can be found in the Guidebook for Employment of Domestic Helpers from Abroad. Moreover, you should take into consideration any additional costs that you may need to incur in hiring a foreign domestic helper (e.g. air fare, costs of health check).
Guidebook for Employment of Domestic Helpers from Abroad
http://www.immd.gov.hk/ehtml/hkvisas_5.5.htm