Approx @2k for a new PVC one and they may throw in a post box.
Be very very careful who you choose, a lot of con men put there. They buy from factories in the North (some are OK, some are no so good) and install themselves. If they are one of the many chancers around then your new door my not close properly (ever), leak, look bad......)
So look at neighbours, ask, especially if they have had many teething problems (bad), any problems getting them back to fix, any other problems.