There has been a thread on here very recently Protector Prices - last post was as recent as yesterday - just go back a bit, about 20 lines down.
I posted that following the guideline on the container it worked out at 68p per sq metre.
You decide what to charge from then on - personally I would be charging £2 /sqmetre if done after the crpet cleaned. More if it was the only job at that place.