https://www.cathaypacific.com/cx/en_...ular-seat.html
Checked with a dummy booking it's CAD57.79 for YYZ-HKG and CAD17.47 for HKG-KIX (i.e. pay per sector/flight) for a regular seat, the above terms also states it may fluctuate as converted from USD. If any sector is priced above Economy save (i.e. YBHKMLV) you can get free seat selection for that part and optionally pay for the Economy Save part.
I think they'll hold some seat till T-48 but it's upon you to buy for sense of security.