Costs involved - partially steep site, partially-stilt home construction

Hi all,

After 10 years away from this forum (and a defunct email address in the middle stopping me from re-using my old username) & 10 years happily living in the home we built then, the time has come to move on, and we now have quite different desires ...

... and said desires have brought us to looking at a rather mountainous north-facing ~1000sqm site with views over the river & which couldn't be built-out.

HOWEVER the problem with this site is the fact that it's so steep ... in parts. There's also a quite large flat section, so part of a home you'd build on it would be on flat land, but some portion of it would need to hang out into the air & be supported by stilts/etc that'd need to be at least 10m tall (maybe more).
The way the block slopes downwards towards the river, maybe the first 5% has a rock-face you couldn't do anything with, then it's flat for probably 70% of the block, then there's a steep sandstone rock-face which probably deserves the name "cliff" ... and I haven't even seen what's below that, as you can't really get a view of it without a boat or a drone with a video-camera.

There's already a substantial new driveway leading to it (a bit of an engineering feat by itself), although I doubt you could use that for deliveries by semi-trailers or pan-techs so there's already one extra cost which didn't occur to me before I started typing this ...

Now obviously you'd need your home design done in a bespoke way to suit the block, and I know from experience that getting a non-project builder to do our current home would've cost us about $1.2m 10 years ago. So I'm going to extrapolate that out to say $2m to build now.
Just to give an indication of my expectation for costs for a 5-bed 3-bed 4-car brick home on a block which only slopes maybe 1.5m from side to side & less from front to rear (but which is spec'd to the level we would want - eg. aluminium-framed thermally-broken double-glazing, tiled floors throughout, lots of cabinetry for decent storage, etc).

Is it absolutely & totally "how long is a piece of string" in terms of what costs would be to be building with part of your house probably sitting on a pretty flat piece of land but with sandstone directly underneath, yet the front of the house needing to be suspended like the house in Lethal Weapon 2? I assume it depends on the soil & rock on that block in particular, both on the flat part and also what that "cliff" is made of, as well as what's at the bottom of that "cliff".
I mean if it can cost half a million just to make a 3-car garage sized basement, I imagine this sort of thing is likely to blow out by a LOT more?
Or is it not actually THAT expensive?