I suggest that you contact National Grid to check for any incentives they might be offering for gas conversions. It is in their best interest to help you become a customer. They paid for for my new boiler and brought the gas line up to the house (back in 2006).
A few months ago, I got estimates from a couple of different companies in Staten Island and they were all around $8,000. Because I needed it done immediately, I went with the company that had the equipment on hand. The others had to order equipment.