Depends on how competitive the keywords you want to target are. Also depends on you current SERP position and your desired position.
There are two types of SEO: On site and Off site. I guess you are looking for Off site optimization (to make your website more visible in search engines), that is usually pretty expensive (some US SEO experts charge $120/h or more, people outside the US are usually less expensive).
I recommend taking a look at SEOmoz: seomoz.org/marketplace
We do both onsite and offsite optimization of websites. Do you use MSN so we can discuss further regarding your SEO requirements?
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Not sure how much people charge for this or what's involved at this stage - I'm open to consultation on various ways this is accomplished.
A colleague of mine and I are experienced and skilled in SEO/SEM. We can guarantee certain things while other things no one can guarantee. If you are serious and prepared to make a nice investment, send me a private message and I'll give you my email. We can setup a time for us three to chat.