Toronto is trying to control the soaring costs. A new report commissioned by the Residential and Civil Construction Alliance of Ontario (RCCAO) advises planners to cut the costs by design. The report called 'Station to Station. Why Subway Builing Costs Have Soared in the Toronta Region' has been published on April 22, 2020.

Toronto saw major projects cancelled in recent years due to cost cutting. Even inflation-adjusted figures show that the costs for subway lines per kilometer has increased several fold over the years. Now the city is looking for ways to keep the costs under control. Unlike material, labour etc costs, some of the cost components have been identified as the "controllable costs". One of the is the tunnelling method. Report says, costs can be kept low by using cut and cover tunnelling method, instead of TBM or Drill&Blast. Report also says, wherever possible, keeping the subway line over the ground will keep the costs down.

Follopwing are excerpts from RCCAO's report (non-Italic are my notes):

From a construction perspective, the biggest cost-increase factor appears to be station depths predetermined  by  tunnelling  choices.  Some  TYSSE  stations  (a recent project in Toronto area) are  seven-storey  underground  buildings  whose  only  purpose  is  to  move  people  between  the  surface  and  platform  levels.  Twentieth-century  stations  were  much  less  expensive,  largely  because  the  TTC  preferred  to  use  shallow  cut-and-cover  tunnels,  open  trenches  and  above-grade  alignments,  methods  that  are messier, but usually faster and much less costly. The TYSSE stations were deeper and more costly than any previously built in the GTA, ...

Report comes up with 10 key recommendations. I take 3 of them here. To read them in full, please check the report (again, the original text is in Italics).

- Minimize the use of tunnels and keep tunnels as shallow as possible. When there is a need to go underground. Best practices from around the world include  cut-and-cover  tunelling  and,  in  less-dense  suburban  areas,  at-grade  and aboveground corridors. Toronto once accomplished a lot at good prices by relying  on  these  construction  methods.  Digging  deep  is  sometimes  essential,  especially in city cores and when connecting with existing subways from below. However, default acceptance of deep tunnels tends to result in prohibitive costs that  often  outweigh  the  benefits  of  reduced  surface  disruption.  The  record  shows  that  rail  projects  have  usually  been  built  faster  and  much  cheaper  when the track-bed corridor is closer to the surface. Shallow stations are more accessible and cheaper to build, operate and maintain

- Plan  for  and  protect  transit  corridors.  As  pointed  out  in  the  “Lost  Scarborough Corridor” (Chapter 8 of this report), short-term thinking led to the loss of a disused but valuable corridor, eliminating the potential for a cost-effective subway option to link Kennedy station with Scarborough Centre. It is vital to create and protect a web of viable corridors in our official plans. All GTA  municipalities  should  be  planning  for  and  taking  steps  to  ensure  that  potential  lines  in  their  areas  can  be  built  quickly  and  cost-effectively  with  a  minimal need for utility relocations, which drive up risk and cost. Discussions with a TTC manager in the 1980s indicated efforts were initiated to minimize utility concerns decades ago on Queen Street and on Yonge between Finch and Steeles, but it is unknown to what degree these corridors were maintained.

- Rethink   factors   that   make   stations   special.   Maximizing   utility   is  paramount,  but  functionality  doesn’t  have  to  come  at  the  expense  of  architecture,  art  and  aesthetics.  Designs  that  aim  to  be  uplifting,  however,  need  not  be  monuments  to  architects’  egos.  We  could  seek  out  relatively  unknown  talents  through  design  competitions,  though  we  might  also  take  another  look  at  the  benefits  of  standardized  stations  –  and  not  just  in  terms  of cost savings. Stand-alone stations with green roofs may be much less green than stations designed and developed as part of mixed-use buildings from the start. Bringing in natural light may be a stated guideline desire, but it’s costly if stations are deep. Pillars need not be undesirable (see the refurbished Museum TTC station, or Moscow’s system). If more subterranean space is desired, make it  horizontal  at  platform  level  rather  than  great  vertical  distances  that  have  to  be  conquered  with  stairs  and  long,  costly  escalators.  Vast  tiled  walls  can  be gallery space for artists or archival photos from the station vicinity’s local history. In short, pleasant stations don’t have to be costly, but spartan ones will be if they’re unnecessarily deep.

You may want to check the rest of the key recommendations as well as the report in full. I have found it an easy to read and enlightening. Naturally not everything is new. It is a very well written tutorial with good examples, if you like:

https://rccao.com/research/files/RCCAO-STATION-TO-STATION-REPORT-APRIL2020.pdf

I have taken the 'Figure 2' above from the above mentioned report.