On costs, it should come to around H$460 to terminate/rehire a helper from the Philippines. The HK govt will charge $160 for the visa processing and the Philippines consulate charges HK$297.50 to notarize the new contract. (This is if you do all the paperwork yourself. Agents will obviously charge a fee if you go through them.)
There is no need to factor in the costs of an airticket as the HK govt is allowing "deferred home leave" for early termination/rehiring so that under this "advanced contract renewal arrangement, helpers are not required to leave Hong Kong after the termination of the existing contract."
Info is posted on the govt website at this link:
http://www.immd.gov.hk/ehtml/fdh_acr.htm