Hi Stuart,
I charge £75 per sq mtr for printed vinyl on flat panel ACM and find this to be a great price for both myself and the customer, it is also very easy for quoting jobs. I have done this for all my substrates, it saves making up prices off the top of your head and pondering for a while before submitting a quote, deciding on if you are cheaper or more expensive than other companies.
It does depend on the market place you are in as to how competitive you have to be. Rent, rates, buying power, time and competition etc all play a major part in my quoting system and all these factors may be more expensive where you are.
Andy