I don't think Price is a big factor here. Fighter jets are expensive, they are broadly similar in purchase cost. The F-18 is probably cheaper to operate currently. But it has weapons and mission capabilities we need. The F-18 is a USN aircraft, the F-35A is a USAF aircraft. F-18 has LRASM and Aim174 and is the only aircraft with those capabilities.
I personally think the fully 5th gen fleet idea is dead. The US and basically every other nation is moving away from it. 5th gen gives compromises, even if it's just cost, development delays and complexity. Again, in peacetime, sure, wait for the 5th gen fighters, but that isn't the situation we are heading into. In 1939 should we have not purchased fighters, and waited for jets to arrive in 1950?
I don't see us standing up a 4th squadron of F-35 before 2035, probably 2040+, there is no capability to push that forward. So that takes a lot of money, people, oxygen, planning on something very far off.
The F-18 line was meant to close this year, it was only a last minute special decision to acquire 17 jets that kept the line open till 2027. Perhaps we should try to buy those ~18 aircraft. USN may be happy to do that. USN gets their money back and they can plow it back into subs, destroyers etc. So this is probably the last chance to acquire those aircraft. We can also possibly acquire some of those pilots/maintainers(!). Particularly with the new administration looking at a ~10% defence cut, the current US situation, people may be willing to look at jumping across. RAAF still gets an extra squadron. It is the only way this extra squadron is going to happen in the next 40 years.
KC-30 would be interesting. There is a A330+ that may be worth considering, but 7% additional range/operation cost reduction may not be worth it.
Airbus launches A330 MRTT+ to deliver extended range - Breaking Defense. But again 2028 window is closing/closed.
I worry that both parties and the bureaucrats are travelling along a business as usual. I'm not sure that is the case anymore.
The days of long careful considered acquisitions, for new platforms are pretty much over. We are in a near war period. We buy what is available. There won't be production slots or capabilities, regardless of price, no matter the need. No matter how much we decide we needed it. Our allies and friends will look to us to try to fill their holes in capabilities . Its not just an opportunity cost, its you spend and buy this, or you buy nothing. Putting aside a peer conflict with China. I'm not sure we will be able to regularly buy weapons from the US, like we used to. They are likely to have many problems including programs finishing, programs cut, programs not starting. Look at the US 6th gen programs. Look at the Constellation class.
Its entirely possible that F-35 production or development will cease/halt during the next 4 years. It could also just become unavalible for sales to Australia. Unlikely, but it is entirely possible. Its also possible it gets further delays and production problems. I don't think that is something we have seen in the last 100 years of acquiring weapons from the US.
We often talk about fighter jets like they are family heirlooms. They are consumables. Big expensive, rare consumables. In a conflict they will be used up and lost doing their job. They consume parts, and having a junk yard full of decades of production is useful. We could buy airframes of F18 from boneyards today, ship them to Australia.
Even just continuing our P8 patrols, they will need escorts going forward to be credible. Even when operating in our own EEZ. That is a whole bunch of extra flight hours, China will try to wear us(and allies) down just through OP. Realistically P8's won't be doing those missions for ever. There will be fighters intercepting fighters in countries EEZ and territorial zones. Maybe we should start a bone yard of 737-700 and 737-800 aircraft to support P8 and E7 ops.
When we talk about increasing GDP spending on defence, it needs to be capability right away, not 20 years in the future, spending a whole lot of money paper work won't help our fight in 2028. We need to shift our mindset.