Jeebus!! $600 is a lot of money to pay for an essay when you could've gotten your professors, family, friends, university writing center to read over your drafts for FREE!
I'd get people who know me best, especially professors who are familiar with my work and can look at the essay from an "adcom's" perspective. Especially advisors in the know...they know what qualities adcoms are looking for so they can help you play up the traits in your ps that fit with the schools you're applying to and whatnot.