The cost for rail and roads is about the same. This is more expensive because it isn't on the ground. Building a road on a bridge is vastly more expensive.
Buses are great for roads where there isn't much traffic. However if traffic is bad you have enough demand that a train is better investment.
I am strongly coming to suspect that a lot of the talk about buses being all you need is fueled more by a sort of NIMBYism than by a fully-informed position. Buses are great and good transit systems need more of them than they do trains, but an ideal urban transit system has both.