Search Engine Rank Cost Advice

I have been looking into what I can do to boost the rank of sites that I create for clients but I do not really know much about search engine optimization. I have been searching and finding what tips I can (I know there is a lot out there) but I have a current potential client that is thinking about selling her clothing online (she owns a boutique) She wants to make sure that she ranks high by searching for particular brands she sells, etc which is a very competitive market. She is asking me for advice on cost - as in will she have to pay someone or a company to work exclusively on this alone? I am not sure what to tell her, so any advice from someone who has experience with this is much appreciated.