In today's political climate of artificially tight budgets, it's far too easy to fixate on costs and nothing else. Comparisons among light rail systems in different cities (or even in the same city) are almost always rendered invalid by the wide variations in service outcomes and specific construction requirements. Costs are impacted by many things beyond distance and vehicle type, although these are often ignored by those trying to score cheap political points. The planned service characteristics and facility layouts have to work with the existing land use, streetscape and utilities. If you want to build and operate high quality transit service, you need to invest a lot in infrastructure, so the capital construction cost will be higher than the cost of low quality transit without new infrastructure. That decision should be based on what kind of transit is necessary to meet the current and future demand for transportation, is very specific to the particular site. Remember also that higher quality transit reduces ongoing operating costs. A fast subway yields great savings over a slow light-rail line that may be cheaper to build, just as implementing so-called "bus rapid transit" wastes far more operating money than doing it right.
But just looking at costs – and trying to minimize them – is not a good way to plan a transit system. You have to look at the benefits too – and there are always judgements that are going to be made. ... Look at the Canada Line. It was built down to a price, not up to a standard. It is therefore less safe than it could be. There are no platform edge doors, which are standard for new automatic train operated subways elsewhere. It is inconvenient with only one entrance for each station, forcing passengers into crossing the road on the surface which is also a safety concern. It is not going to be big enough if Vancouver actually achieves its 2040 goals: the platforms in the stations just cannot accommodate much longer trains.
Focusing on the cost of capital construction means devaluing the service, providing lower quality transit than should be provided, and spending more money in the long run than is necessary.