I'd say about 6K. There shouldn't be too much damage to the walls apart from where the pipes travell upstairs/downstairs, which should really only be in one place. It also depends where your gas connection comes into the house and what type of boiler you go for etc. You can run the pipes along the bottom of the wall, which will need some kind of covering with extended skirting, otherwise, if you put them under existing floorboards, which would look neater, it means cutting/ripping floorboards.
Best to get one of the gascompanies to send out an engineer who will talk you through it all.