The relative costs of different technologies of transit are not fixed. Although there are some rules of thumb for the ratio of tunneling cost to above-ground transit cost, the actual ratio depends on the city and project, and this would favor the mode that’s relatively cheaper. Likewise, the ratio of operating to capital costs is not always fixed, and of course long-term real interest rates vary between countries, and this could again favor some modes: more expensive construction and cheaper operations favor buses, the opposite situations favor rail.
In general, els cost 2-2.5 times as much as at-grade light rail, subways 4-6 times as much, according to Table 6 in this Flyvbjerg paper; Table 5, sourced to a different previous paper, estimates per-km costs, and has ratios of 1.8 and 4.5 respectively.
However, specifically in Vancouver, the premiums of elevated and underground construction appear much lower. The cost estimates for rail transit to UBC are $2.9 billion for an almost entirely underground extension of SkyTrain and $1.1 billion for at-grade light rail along Broadway, both about 12 km. Elevated construction is in the middle, though closer to the light rail end: the estimates for the two all-elevated SkyTrain extension alternatives into Surrey are $900 million for 6 km for rapid transit alternative 3 and $1.95 billion for 15.5 km for alternative 1. The under-construction Evergreen Line, which is 11 km long of which about 2 are in tunnel, is $1.4 billion.
In the rest of Canada, this seems to be true as well, though the evidence is more equivocal since the projects that are considered above-ground are often elevated rather than at-grade. The Canadian above-ground projects that Rob Ford’s Eglinton subway is compared with are not wholly above ground. Calgary’s West LRT, which with the latest cost overrun is $1.4 billion (a multiple of the preexisting three-line system) for 8 km, includes a 1.5 km tunnel, a short trench, and some elevated segments. Edmonton’s North LRT is $750 million for 3.3 km, of which about 1 km is in tunnel and the rest at-grade. But while it’s hard to find the exact ratio because of those mixed projects, the costs are not consistent with the ratios found in Flyvbjerg’s sources.
Outside Canada, those ratios seem to hold up better. American above-ground transit projects, such as the Portland Milwaukie extension and the Washington Silver Line, are as expensive as Calgary and Edmonton’s light rail, but American subways are much more expensive than Toronto’s Eglinton subway ($325 million/km, 77% underground and the rest elevated): Manhattan tunneling is more difficult, so its $1.3-1.7 billion/km cost may not be representative, but conversely, BART to San Jose’s $4 billion for about 8 km of tunnel is for tunneling partially under a wide railroad right-of-way, with no crossings of older subway infrastructure as is the case for Eglinton at Yonge.
Conversely, French tunneling costs are comparable to or lower than Canadian ones, but at-grade light rail is far less expensive than in North America. The RER E extension was at least as of 2009 budgeted at €1.58-2.18 billion for 8 km of tunnel (see PDF-page 79 here; this excludes €620 million in improvements to the existing commuter lines the tunnel will be linked with) – somewhere between the per-km costs of Vancouver and Toronto subways, but in a much denser environment with more infrastructure to cross. But the cost range for Parisian trams is much lower, about €30-50 million per km, in line with the subway:tram cost ratio of 4-6; the cost range in other French cities tends to be a little lower.
What this means is that in Canada in general, and in Vancouver in particular, questions about what mode to build should have higher-end answers than elsewhere. It doesn’t mean that the Eglinton subway is justified, but it does bias suburban rail lines in Vancouver toward elevated SkyTrain extensions rather than light rail, and inner extensions toward SkyTrain subways. For the same cost of building a subway under Broadway, Translink couldn’t build too much additional light rail; it could build two lines, say on Broadway and 41st, or maybe three if both non-Broadway routes are short, but certainly nothing like the entire network that SkyTrain opponents believe is the alternative, citing European tramway construction costs.