Just some issues that you may consider.

What it costs depends on what you use (4k AC or a 8k AC,etc), size of your house, structure of wall and floor, and how well you want to get it done.

Issues:

Locate the furnace and vent pipes so that the distance to each room is about the same. Otherwise it is hard to get the pressure balanced and you will have some hot and some cold. Your cotractor may plan it to his convenience, but not to your interest, for example, running the pipes to the attic and go down to each room is not a good idea(that's why nobody does to a new house). The pipes will be too long and you will not get enough heating upstairs. The same with returns.Unless you have the pipes insulated, you will be losing energy to the attic.

Since you will take down the steam heating, then there is the issue of fixing up the holes on walls and floors. I can't see how you can do nicely without doing considerable amount of painting, floor, or carpet work.