The problem with hiring a SEO is that it's difficult to put a quantitative value on ranking for a keyword.
There was a big scam at DP about a year ago where people were paying $100 per million results to get ranked 1st page on a keyword. The results were 100% guaranteed. In the end people were asking for their money back and the SEO couldn't refund. He said that he had wired the money to Beruit Lebanon to someone that he didn't know. Some people lost in the range of $7,000 or more.