It is not cheaper for building.
1000 meters long two tracks metro line on the 20 m underground costs 20 332.
The same road, 1000 m, 20 m underground, usual road, one way bus line = 6 039, to put tram tracks takes 2 001 money, thus we have 8 040 for one direction and total 16080 with way back. If build not usual road but expressway, the road cost is twice higher.
So only building the road is compared by the price with building metro line. And consider the need for creating entrances, that needs to buldorize buildings, this pre-metro line takes much more money.
BUT!
Is is little more estetic.
It can be used by all types of transport (bus, trams, trolleys if you put the lines).
You can make express bus lines through whole city not thinking about road traffic and without creating a lot of change-line stops with hundreds of people migrating from stop to stop.