SEO is a technical area, so it’s important to be absolutely sure you know where your money is being spent and that results can be reviewed properly.
Google does not sell better rankings within their search results and some seo’s will promise to get you onto the top of Google, but instead, place you in the advertising section, rather than the search results.
You need to ask any SEO you are considering, which fees go towards the permanent inclusion within search results and which apply to temporary advertising.