Maybe because there is a huge variety of sitiuations depending on the local infrastructure, gas network, age of pipes, pipework-valves-ect in house? We looked into it in Sheffield 13 years ago, would have been £1200. Old house, gas main outside.
I'm sure you're exactly right bike, that's totally why there are such a variety of answers. The problem is that they don't always explain the situation so I'm not sure if it would apply to us or not. (Someone said 20K!)
Old house, gas mains outside sounds similar to us.