I used the writing center on campus, but the biggest help to my essay was when I found an old English prof who's class I had done well in, and asked him to go over my essay with me. Every time I'd proof it, I'd take it into his office and he'd go over it with me and we'd talk about punctuation, grammar, content, getting my point across, etc. And it didn't cost me a dime. I also think that because he remembered me from his class, I got a lot more personal attention and empathy from him than I would've from a service. So I would recommend finding an old professor/TA first, then go to the campus writing center. Only as a last resort would I hire a service. They're going to be expensive and probably won't know any better than some TA.