Rheem-Gas Furnace

Question

I have a gas furnace that will not ignite. I have attempted many times to start/ignite it using the manual suggested sequence, and I have been unsucessful. In diagnosing the problem(s) the technicial manual flowchart suggests that I replace the vent pressure switch. How do I not know I need to replace the igniter. What tests can I do to ensure it is.
If I let a proffessional trained technician perform the work, How much should I expect to spend?