The cost to select a seat or the fare for that route?

Just go to the CX web site and enter the dates and you'll see the price. It's not clear why you cannot find the cost because you have not said what happened or when you want to travel to Osaka. You could use a travel agent as well, as their fee is modest (like $40) and they can sometimes find routes and fares more easily (and more easily than your luck so far).

As to the latter I bet pretty slim but on these basic fares you are able to book seats early with a fee, If it's important then pay or be Johnny on the spot 48 hours out.

FYI I did a quick test booking out April 3 and back April 24 and basic economy was CAD$1,624 per person round trip. Out at 0145, arrive HKG 0500, leave HKG 0755 and arrive at KIX 1255. A day later of course.

Seats assuming you found a fare are according to CX:

We’re now extending regular seat reservations to passengers travelling on Economy Super Saver, Economy Save, or fanfares tickets – for a nominal fee, depending on the origin and destination of your flight, from HKD60/USD8 to HKD330/USD43.

For that distance it's a safe bet it's US$43. The fare which includes seats, has a lower change fee and more Asia Miles points costs CAD$250 more per person.

For seats which won't kill you assuming you are taller than 5 feet:

If you need more leg space when you fly, you can stretch out with our extra legroom seats in Economy Class – available for an additional fee from HKD180 / USD23 to HKD1,550 / USD199, depending on the origin and destination of your flight.

Again I'd bet on US$199.