You can't. The tunnel is cheaper because it's not for a subway. The tunnel for this 'Dugout Loop' is a simple tunnel lined with concrete and possibly some lamps for lighting. That's it. It doesn't require any of the expensive things that a subway requires like tracks, electrical systems like a third rail or overhead cable, fire suppression systems, etc. The dugout tunnels can probably also be steeper and have sharper angles, which allows more possibility for routing that a subway wouldn't be able to do (vehicles on train tracks can't go up very steep inclines or take very sharp corners).
Another substantial part of this is that the Dugout Loop vehicles ascend to the surface through an elevator or ramp, which allows the station platforms to be built above ground. The platforms can also be smaller (they only need to fit a couple car-sized vehicles, not an entire train). This reduces cost substantially, as building out a large underground train platform is often one of the most expensive parts of a subway build.
Another substantial part of this is that the Dugout Loop vehicles ascend to the surface through an elevator or ramp, which allows the station platforms to be built above ground. The platforms can also be smaller (they only need to fit a couple car-sized vehicles, not an entire train). This reduces cost substantially, as building out a large underground train platform is often one of the most expensive parts of a subway build.