Matt cleaning is hard work, you are lugging big equipment about, its gonna take at least half a hour to clean a room, as you know you need expensive equipment to carry out the task. Its time consuming and at the end of the day you feel knackered.
With protection you stick a bit in a bottle spray it on the carpet and its done 5 minute job, there really is nothing to it.
Also with regards to people charging 50% on top for this service, to work out how profitable that is all depends on what the cleaner is charging for the clean in the first place. Obviously if there charging £10 to clean the room theres no profit in protecting at a extra 50%, but if there charging £100 then there is profit to be made in £50.
I only charge 33% on top as its easier to sell it at this price, and i make roughly double what the protector costs me probably a little more. So you buy a bottle of protector for £30 spray it on the carpet and walk away with £70-£80 not loads of money but its more than your making by not selling it.
My advice to you is just buy a bottle and try it out it really is easy money mate.