I would check other things first before condeming the starter. Check the battery has enough juice in it and that it is not knackered. Then check the earth points in the engine bay. Any high resistance will decrease voltage.
There is a bearing in the bell housing that the spigot end of the starter motor fits into. If that bearing is worn, damaged or missing, then that will also give the same symptoms of a discharged battery or faulty starter motor.
If all that fails, then the local breakers yard will be the best and cheapest option, or you could get your starter repaired.