Uh uh. That 80% figure includes suburbs, exurbs, and all manner of low density residential settings where mass transit is uneconomical. So now what?
Besides, if you care about costs then suburbs/exurbs are fundamentally uneconomical in general - most of the cost of housing comes from the land, of which suburbs are fundamentally wasteful. Literally everything there is (relatively) uneconomical, and is only widespread because the alternative is literally illegal.
Cheaper like cheaper or cheaper like more expensive, but subsidized by taxpayers who don't use it?
And, honestly, I think some number of people would be willing to move into denser areas if there was good transit, so it's a problem that might work to solve itself in time.
(and recommend tooling - PostGIS is likely ok for this kind of thing without too much effort (fingers crossed))
It's a learning curve if you haven't done such things before, perhaps worth it if such questions interest you.
You'll also be doing some seat of the pants Fermi estimations so it's be handy to run your thoughts past others that are into that kind of thing.