1 Top vented gas fireplace inserts cost $750 to $3,000, and installation adds another $500 to $1,000. 2  Direct-vent gas fireplaces cost $1,000 to $3,500, plus $500 to $1,000 for installation. The cost of a ventless gas fireplace is $500 to $2,500, and while there are additional installation costs, you may pay extra for finish work (such as a mantle, marble surround, etc.).