First of all, it needs to be done anyway, regardless of whether you have a PPC campaign going or not.
There is a credibility issue with paid ads. Viewers are more likely to click on a general listing than an ad on the top or side of the search page.
In general, you will pay less overall for the organic optimization than for bidding on keywords.
You should have your web site optimized by a professional because there are so many tools available and knowledge of their usefulness is critical. Unless of course, you want to purchase them and have the time to learn how to use them yourself.
It isn't just formulaic, there is a certain amount of creative judgement involved. Things change by the minute so a professional, if he or she is good, will be on top of current trends.
Why not try organic first before you spend money on PPC? You may get the results you want. You can always add the paid campaign to increase what is happening naturally.
