Sorry cant help with price, but can give you details of where to find help:
If you call National Grid's New Domestic Connections on 0870 903 9999
They will send you a form to complete and return it. Once they have received your application they will send you a Quotation for the work, together with an Acceptance and Payment Form for you to sign and return.
As you require gas supply for a newly built property, they will ask for a scale plan of the building within the plot, together with a location plan of the plot and a clear indication of the plot or property number.
A quote will be provided to you for a new gas service pipe only. However, due to industry arrangements, you will need to contact a Gas Supplier to arrange for the installation of a meter and a gas supply contract. Your gas supply will not become operational until this is completed.
National Grid Website: http://www.nationalgrid.com/uk/Gas/Connections/
Hope this helps!