Hi purplekitty,

Speaking as someone with two young children, and a decent disposable income, there is no way I would pay >$3000 so an infant could get their own seat. I say this without the experience of traveling with a lap child all the way to NY (long flight! good luck!), but I have done shorter flights to/from Asia with a little person on my lap.
A few comments/views:
* flying with a five month old isn't going to be much fun (except when they are asleep) irrespective of whether they have a seat or not; I'd be approaching the journey as a 28 hour challenge and accept that there is a limit in what you can do to prevent that.
* if traveling economy, you can raise the arm rests between seats; this can make things a bit squishy but if you and your traveling companions aren't too large there should be enough room for the tike to sit between you at different points during the flight. Is the child traveling with you a sibling and young (<7 yrs)? If so, lifting armrests and sharing space is a real possibility.
* the flight may not be full, and if not, I've found Qantas very good at organising a spare seat when traveling with a lap child (although, this may be a function of FF status)

My thoughts.

Good luck!