Here's an example of some fares out of interest, for dates well in the future (I used March 2020) and using Sydney as origin:
QF saver - $625 outbound, $555 return (total)
Of the $1,179 total, $960 is the base fare and the rest taxes/fees, so the total return cost for a booking with comfort seat would be $2139 .
Note that QF3 & QF4 only run four times a week.
Jetstar "business" is around $2,628 for similar dates, plus $220 if you want lounge access and Qantas SCs/points, so $2848
Frequency varies by season from 3 times a week up to daily
The final option is Hawaiian, their "extra comfort" seats would not be suitable as it's just extra legroom, no width advantage - like many US airlines they don't have a proper PE and try to claim this "economy plus" (or similarly named) is equivalent when it's definitely not - so be wary of this. Business class is around $6k return. Economy fares are comparable to Qantas, and they do offer the ability to purchase an additional adjacent seat for a second economy fare (I couldn't establish if the taxes are waived, but certain fees certainly should be since it's only one passenger).
Both Qantas and Hawaiian require you to book through their contact centre if you want to use the extra seat - the web site doesn't handle these. Note that Hawaiian is a Virgin Australia partner, so you can get VA points and SC's on there if that helps (be sure to book a VA codeshare through Virgin directly).