you need to research the going rate for your area and aim around that to start with. in my area the rate is £2.50 per square metre for re sanded block but other areas can command a lot more. the best way to work it out i think is to set yourself an hourly rate. you will with experience be able to estimate the time a job will take and apply your hourly rate. say 40 to 60 pounds an hour. often the small area jobs take a disproportionate ammount of time once you take travelling ,setup , tear down times so factor these into your calculations.
Ged