1 Vent-free gas fireplace (no chimney required) with installation: The standard cost for a vent-free, or ventless, gas fireplace is approximately $800-$2,000 installed. 2  This does not include the cost of any gas-line work needed.3  Installation costs for a top-vented gas fireplace: Approximately $300 to $1,200. This does not include the cost of any gas-line work needed. 2  Installation costs for a top-vented gas fireplace: Approximately $300 to $1,200. 3  Direct vent gas fireplace installations: These are more expensive and can cost up to $5,000.