Door prices in Glasgow vary because every entrance is slightly different. A simple replacement uPVC back door will usually sit in a different price range from a composite front door with decorative glass, side panels or premium hardware. Larger openings such as patio doors and bi-folding doors can vary even more because size and frame system have a major impact.
The final fitted cost may depend on the door material, frame, colour, glazing, locks, handles, threshold, disposal of the old door and whether any making-good is needed around the opening. This is why guide prices are useful, but a proper quote normally needs survey information.
When comparing quotes, avoid looking only at the lowest headline figure. Check what is included, how long the guarantee is, whether the installer is local, and whether the specification matches the door you actually want.