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It would most likely be less expensive, especially since the LRT is being extended. Here's why:
Operations
Since rolling stock facilities need to be built for the SLRT that adds to costs (either that, or deadhead trains all the way to Mt Dennis, and that's extremely inefficient). The presence of a new yard will increase operating costs in itself, so that's already a huge bill to swallow.
Traction
Contrary to what I believed earlier, subway traction power requirements are exactly the same as what would be needed for as the proposed Scarborough line. Here's why: A typical Flexity streetcar weighs 48 Tonnes, and 4 Flexities are supposed to make up one Scarborough LRT Train. Compare this to a 6-car Toronto Rocket train: 205 Tonnes. They are almost exactly the same mass. If we assume passenger loads are the same for each mode of transit, their effects are negligible. According to the theory of the conservation of energy, Work (The total amount of power required across a distance) is based on the force required to move the object times the distance. Since the distance would be longer on the LRT line, and the force required to move the LRT and subways would be the same for both vehicles (F=ma=(~200,000kg)(acceleration of train required m/s^2)), traction costs would, in fact, be greater on the LRT line then they would be on the subway line since the distances are greater on the LRT line if we assume the terrain is flat. Note, work calculations are actually a lot more complicated than this because of circular energies of the wheels, but doing advanced integration here would probably get me banned.
However, (skip this if don't give a shit about physics) this is only assuming no inclines are present in either line, and we know for a fact that the depth of the subway is to decrease significantly over the course of the SSE. For every train travelling to Kennedy, they will also have to travel up an incline. For this, we have to add the initial work required to move the train along the line to the work required to move the train up. This can be represented by an equation for potential energy, U=mgh. If the train is to have to move up an incline of 20m, 40 megajoules of extra power are required. If this incline occurs over 2 minutes of travel time, 330 extra kilowatts of power are required to move that train up the incline. This is a difference of 550 amps of power per train. This is only going up the incline though. Going down the incline, the train does negative work to travel deeper underground. Since regenerative braking gives back about 30-50% of the 40 megajoules of energy required to move the train up the hill, the net energy difference as a result of this incline is between 20 and 28 megajoules per train. Let it be known, the terrain for the SLRT is not very flat, and the line goes up and down inclines as it switches from grade level to elevated.
Operators
Operators are the highest cost of a line. Since more drivers will be required to move the smaller trains to the STC, those operation costs increase
Since the line is a new standalone line with a different technology, it must have different route planners and different operation executives. These are the most expensive employees to pay, so operation costs there are significantly higher. Compare this to that of the Bloor Danforth line, where fewer operators are required to run trains, they are already employees of the TTC, and executives are already running a line and at worst will need small raises.
Maintenance (Trains)
Since LRVs have a statistically higher rate between breakdowns (15K km vs 600K km, so LRVs are 40 times as likely to break down as heavy rail vehicles), they require more deadheading and more Mantainence staff. These maintenance staff would also have to be based out of the new yard, or trains would require significant deadheading, meaning costs would increase no matter where maintenance takes place. Lost revenue also has to be considered when trains are taken out of service.
Maintenace (Infrastructure)
This one's hard to say, since the LRV would be elevated and longer than the subway. On one hand, concrete liners aren't an issue. On the other, pantograph carbons, & pantograph lines wear out quickly and need replacement more often than third rails etc. The line is also aboveground, adding to operation costs since they are susceptible to the elements.
Inefficiencies
It will take longer to take the LRT from Kennedy to the STC (where everyone travels to anyways). This must be accounted for with lost revenue. Factor in a transfer and the line pays for itself after 60 years. You must also factor in delays associated with running a line outside, where it is at the mercy of the weather. However, when you consider potential bloor danforth delays, this can potentially be ignored depending on future operation of the line.
