How much does a new gas furance cost?

I assume we are talking about replacing an old existing unit. It depends on a few things. Like cars, gas heaters come in basic models with basic efficiency or more expensive models that are more efficient.

Then there's installation, piping, wiring, duct work & inspections from local agencies depending on your code where you live. Where does it vent to-the chimney or directly outside.

A basic mid 80's % efficient furnace that vents into a chimney-probably about $3000-$4000 delivered and installed.

A more expensive, 90's % efficient that vents directly outside in PVC pipe-probably $4000-$5000 delivered & installed.

Piece of advice though. Get some licensed & try to get someone who gives you 1 price delivered & installed.