I think you don't need a professional services for your PS.
Of course it's important that people you trust read and correct your PS if necessary but you should be able to write your own statement.
There are so much resource available which must be enough to put you on the right path. They won't tell you what you should write but there are different tactics and prompts that you should be able to use.
You must check writing center of colleges like Perdue OWL or Berkeley College career center. Also many educational websites are providing resources. I came across to this resource recently which gives great tips about writing a PS http://www.gradschools.com/article-d...-statement-114
If you have money to throw away you can always try the professional services