How can they be cheaper? You still need to install all the overhead wiresm this only works if you assume they the routes never change. And will require an absurdly huge initial investment. Might be better/cheaper to just see if batteries get better or if more quick charge points can he installed.

The vehicules are have less components thus are both cheaper upfront and cheaper on at operation by being lighter and requiring less maintainance. Because of that they are on average bigger so you can have a smaller fleet. Also installing overhead cables isn't such a big investment (in the face of related urban investment needed anyway like road maintainance, street furnitures and traffic/lanes shaping). Remember that tons of cities install christmas decorations with similar overhead wires just for one month every year.

I wonder how many more passengers a trolley bus can carry, compared to a battery powered bus of the same size