It is impossible to say. You will have to have tests carried out to make sure your foundations are strong enough to take the extra weight. If they are not - it will add considerably to the cost.
Also, your roof joists may need altering - another large cost. And the floor will have to be strengthened.
Do you want dormer windows or velux windows? Is there room to provide the extra staircase and firedoor? Will you want to install a bathroom up there?
Get quotes. Find local people who have had recent conversions and ask for recommendations.