Jason,
Usually, it is cheaper to have a starter repaired at an automotive electrical shop than it is to buy a new starter. As for ordering a new one from Amazon, since Amazon has many vendors, there is not way to answer the question with certainty, but most Amazon vendors are very good, and Amazon has a very liberal return policy.
Are you sure that you have a bad starter? On many Onan generators, the starter circuit runs through the generator control module as well as a starter solenoid. Just because you push the starter button and nothing happens, it does not necessarily mean that the starter is bad.
Gerald