Hi,
As limescale is mainly calcium carbonate and magnesium, all you need is somthing with acidity to breakdown the compound.
Therefore, the simple use of distilled vinegar is ample.
For shower doors, and general cleaning we use a ratio on 10 parts water/1 part vinegar with a splash of lemon juice.
For heavy duty use, use a 50/50 dilution. Obvioulsy, it comes with a tad of elbow grease.
For shower heads, try saturating a jeye cloth with vinegar and simply wrap it around the shower head, allow some dwell time (the longer the better) and you will be amazed at how well this works.
As for cost comparison shall we say with the 99p lidl W5 (which I have tried) it works out for the 10/1 ratio 0.03p for a 1 litre spray bottle
(also excellent on glass/doors/windows.
We also use it for wood furniture with excellent results, you simply mix the distilled vinegar equal parts with lindseed oil.
Hope this helps.
Paul