You shouldn't expect a price without the thatcher seeing the job, but mostly it depends on the size. The unit of measurement is a thatcher's square, which is 10 feet by 10 fett, i.e. 100sq.ft.. So I suggest you measure the roof and tell us the size. Then there is the material. The two commonest materials are reed and longstraw. Since your house is listed the conservation officer needs to be consulted. You may not have a choice as to which you use but reed lasts much longer than straw. What is there on it at the moment - you will probably have to have the same.
You shouldn't expect a price without the thatcher seeing the job, but mostly it depends on the size. The unit of measurement is a thatcher's square, which is 10 feet by 10 fett, i.e. 100sq.ft.. So I suggest you measure the roof and tell us the size. Then there is the material. The two commonest materials are reed and longstraw. Since your house is listed the conservation officer needs to be consulted. You may not have a choice as to which you use but reed lasts much longer than straw. What is there on it at the moment - you will probably have to have the same.
Biff is quite correct. We cannot give you a useful guide without much more info and in any event, the actual cost will depend on many factors including how busy the local thatcher is at the time.
The more complex the roof the more it will cost. Therefore if you have several dormer windows and a couple of chimneys it will cost quite a lot more than if it is a simple double-pitch (up and over) roof.
How long it lasts depends on uch a wide variety of factors. You will find that some talk about different thatch materials lasting longer than others, but this is very much in a perfect 'all things being equal' world.
In reality some thatchers are better at some materials than others, some supplies are better quality, etc, etc. Much depends on the preparation and then the skill of the thatcher (watch that you don't get the thatcher start and then his 'boy' do everything else with little supervision - it happens!). The location and orientation of the building can affect longevity.
Sorry to throw more issues at you than answers, but because thatch is an organic material it cannot be categorised in the way that a controlled manufactured material can be.