Chicago is extending an existing above ground train line along an existing freeway (no right of way issues, not a subway, etc) 5.6 miles. It’s four stops and a new rail yard. 3.6B.
Something may have changed more recently. The first stage of the Eglinton Crosstown LRT line in Toronto looks like it will come in at around (converting currency and distances) 9B for 12 miles. Locals generally don't seem to think this represents a success. (Despite being light rail, about 2/3 of this first stage will run underground.)
https://www.transitchicago.com/ctas-36-billion-red-line-exte...
It may be an American problem but it’s certainly not just NYC. But of course most of Waymos market is American as well.