Best thing I did was to read the advice of Nathanal Jones on ACS website forum. Get a decent power washer and either a flat surface cleaner or turbo nozzle.
Then clean your own patio and see how long it takes.
Then price the job accordingly.
Remember to add on time for setting up and cleaning the equipment as well as fuel costs.
There is a company in Ireland that Nathanal and loads of others use including me. The equipment is really good value and is very powerful making the job easier!
Hope this helps.