Huw,
Are you sure it will be a three stage clean?
Commecial build cleans in my experience are a one clean deal, as normally the occupier will want to move in as soon as most of the work is done, might be different in the current climate though.
Regarding the price, depends on a whole heap of things, such as materials to be cleaned, how messy the contractors have been etc.etc.
Dont be afraid to price accordingly!
Regards,
Rob