Some protectors require application onto a damp ( sprayed with water) carpet, others require pre spraying with an acid rinse product. this is no problem. All protectors work out about the same price per sq/m cost , so it depends what you want for doing it. Things to take into account are -
area to protect ie sq/m
is there furniture on it
normal costs of getting to the job
So I would say between £2 - £ 3 sq/m
Mike