There is nothing wrong in pricing per square meter as a starting point, you need to factor other things into the equation like drainage ect.
Everyone cleans x meters squared and charges y pounds to do the job so everyone has a price per square meter, they might not know what it is. This is why I think its laughable when people slag that method of pricing, as it leads to consistent pricing. Ill bet if those that looked at there prices per m2 after the job was done would see quite a bit of inconsistency, which may or may not work in your favour.