You are right and I believe the HSR to Canberra would cost 8 to 10 billion for a 50 minute trip.
I think that the 17,000 hectares would bring in $20b which I believe would be enough to build it.
The airport site area is only 1700 hectares. That might get you a billion or so if you sold it, which, as others have said, might pay for the design and other up front studies for your train line, but won't be enough to get anything built.
I did read it would cost about the same to set it all up as to build the HSR line but that was by Canberra airport so they may be biased.
The HSR phase two report estimated a basic infrastructure cost of around $20 billion, plus around another $4 billion for non-infrastructure costs, for the Sydney to Canberra segment.
When comparing Badgerys Creek with some sort of train to Canberra you need to be very clear around what
is being proposed for a particular option, when
it is proposed to be built, how much
it will cost and who
is going to pay for it.
As currently planned, the initial airport to be built at Badgerys Creek will be nothing like the final size of the airport that one day could be at that site. Initially you might have something like one significant runway, plus some moderate sized passenger and freight terminal facilities and moderate sized connecting infrastructure (e.g. some improved roads). Further expansion, in terms of the second runway, larger terminal facilities and better connecting infrastructure (such as a rail line) can be deferred until the time that the level of demand makes it worth while.
So if you decide to proceed with the airport tomorrow, you are not spending $10 billion or whatever for a full sized airport (see the Joint Study on Sydney Aviation Capacity for some indicative costs), you are spending more like $2 billion to get something that is big enough for today, but designed to be easily expanded. Then over the next three decades or so
, you spend the difference as demand builds up (or, if demand doesn't materialise - you don't spend anything more).
But if you then consider the Canberra option, to provide a similar level of convenience to passengers you pretty much have to build a full spec, double line, high speed railway, with long tunnels all the way into the Sydney CBD, before anyone is going to consider travelling that far just to catch a plane. So if you decide to do that option tomorrow... that's pretty much $20 billion that you have to hand over tomorrow (and that excludes spending any money on Canberra airport itself). Things like rolling stock can then be purchased as demand grows, but the rolling stock cost is a tiny fraction of the infrastructure cost.
But then the kicker... who is going to pay for these things.
As currently planned, the bulk of the construction cost for Badgerys Creek will be paid for by private investors, who will then recover that cost through landing fees, passenger fees and facility rents. The state and federal governments will probably have to pay for some of the connecting infrastructure, such as roads (but they could also be made toll roads) and perhaps ultimately a rail line (which might plausibly recover some of its cost through station access charges), and they have already paid for land acquisition, but the bulk of the spend is not the taxpayers problem.
However, in the case of a high speed train to Canberra, it is pretty much accepted that the only way it will be built is if the taxpayer pays for most of it, without any real expectation of recovering their contribution. Fares paid by passengers on the train are only expected to cover the cost of buying rolling stock, operating the services and maintaining the tracks. Private investors, such as Canberra Airport, might make some contribution to the cost of some stations, but you are at best talking a few hundred million out of a total infrastructure cost of around $20 billion.
In terms of providing a second airport for Sydney, HSR to Canberra airport, as an option in isolation, just does not make sense.