If the construction inflation index is consistently greater than the general inflation index, then it is always cheaper to build now than to build in future.
Of course, it might be more beneficial to direct the funding available today towards another project that will be operational sooner than in 25 years.
But if the future cost of DRL station increases due to 3 factors simultaneously: construction cost inflation, the need to build under /over the existing line, and the need to deal with the built-up intersection; then I would not be surprised if building the box now comes out as a better option.
As pointed out, O & M has to be taken into consideration along with the funds to build it, as well what it does to the system wide at the time.
At the same time, technologically will change over time to the point what you built for today may not work down the road.
Today we look at a single level system when we should be thinking about going double deck, as it will carry far more riders than a single level line. There are very few places you could put in a 3/4 track system to deal with the increase of ridership 50-100 years down the road.
At the same time, you are guessing what a station location may look like to the point you can overbuilt/under built the station box and the protected area for it.
Since there is very little funding for transit these days, the service charges to pay off the cost of building this infrastructure now could be less than the cost to bring it up to current standards than building it new then. This will take badly need money from something that is badly need then.
Take a look at the Queen St line station and you will see it will cost more to rebuild it if the DRL goes along Queen St than what it has cost since when it was first built in the 40's for PCC streetcars. It totally out dated and will never handle the ridership on opening day.
Platforms are a really big issue as they are too small today, as well having next to no good traffic flow to move riders faster to/from the platforms safely.
North York Centre station box was never built when the line was built to Finch, as there was no need for one then. Today it sees over 25,000 riders and will continue to climb with all those new condos being built around it now.