I took an intense writing/editing course during my senior year of college with a professor who has written a leading book on grammar and style. It was a paper due every week plus and additional 20 hours a week of hard-core editing of classmatesí papers. Itís the type of stuff thatíll really hone your ability to edit your own prose.
But regardless of your own confidence level, you should always have your work looked at by fresh eyes. For my PS I cornered some of my buddies from the class as well as the professor. I was lucky to have people around me who knew what they were doing.
But if people donít have access to these sorts of resources, I could totally see why they would drop the money for a good editing service. I could dig $70 to $150 bucks for a solid read-through with line-editing, micro and macro. it can take a good 3-5 hours to really help reshape a 2-page PS. trust me, i've spent that much time on a couple of statements from (now) Harvard-bound lovlies on this board. i'd say it's worth the money to have someone help you go from dull and bland to sparkly and shiny.