As long as you change the oil and oil filter once a year, everything else will not do harm to your engine, but it might not run quite as well. For the price of a service kit, and a couple of hours of your time, you may as well do oil, filter, air filter, plugs (if required) etc too yourself. All easy jobs.
A service generally includes much more than the obvious, so I can understand the cost to a degree. Things are cleaned, inspected, lubricated etc, but none of this will show on your bill, but all take time and therefore cost.