First of all, you need to consider whether or not SEO suits your online business. If you would like more visibility to more clients, then the answer here is very easy. If you are good at implementing SEO yourself, then you can make a instant decision on where to start with your efforts for better online visibility. Otherwise, get started on spotting the most appropriate SEO firm to give you some help.
The first sign of a good search engine optimization service firm is a need to examine your site before they offer a quote. All these firms understand that sometimes, your whole site needs a tiny modification to completely change and become very visible and competitive. Stay away from SEO firms that will promise and "guarantee" you 1st page rankings in almost no time. These companies will most likely use black hat techniques to rank your website. The short-term result is good rankings, which disappear as fast as they came.
In the end, the cost of the seo service has to be affordable and cheap enough for your budget, but well enough to guarantee quality service. Remember that being successful with SEO is an on-going effort and not a one-time activity.