What drives your sales? Is the search ranking important? Of course the Google ranking will be critical to your sales performance. 
If you cant be seen online then your sales will be nil. If you come up first on a google search then you will be in business. The real trick is to get that ranking up and keep it there. That will cost you a bunch and you will have to figure how much to spend. I would say that SEO help is very cost effective and those not savvy enough to use it will fail. Tim DeCapua is a real fan of SEO intervention.
