so a Toronto to Montreal would cost at least 500 million, closer to a few billion

$3 billion on 1.7km of Gardener Expressway repair. $3.6 billion for Turcot interchange replacement. No one bats an eye at those costs.

passenger load which is what makes a transport system economical

Toronto-Montréal is the busiest domestic flight route, followed by Toronto-Ottawa. Add Chicago and New York to capture the two busiest routes between the two countries (both in the top 20 international flights). Plus however many bus, train, and drive.

Edit: just plugged 15 Nov into Google flights; there are 46 flights from Toronto to Montreal that day (Pearson and Island combined).