The advertised cost of an off-the-shelf front door doesn’t usually include installation, delivery, hinges, disposal of the old door and sometimes VAT. This means a door marked as £500 could end up costing double. Doors with side windows and panels tend to cost more to be fitted. Try to get at least three quotes from different suppliers for comparison.
For many period homes, the only option will be to have a bespoke door, due to the non-standard opening size and shape, and this will inevitably add to the cost, with prices starting at £2,000.