I am sure small businesses with no PR agency would go to individual PR agents for a low cost solution. As for the price, PRpro's suggestion is good. You can also check sites like Fiverr.com for idea on the price or some other popular freelancing sites.
@lilybags, yes! I think that will help a lot of confused webmasters or site owners out there in the world of building and maintaining a long enduring ecommerce website. It might also help if you can also include the free ways or low-cost ways to handle that dilemma.
I would say, create a buzz around those books that you have for sale. Get a list of Press Release distribution sites and submit to information about your new book. Another way is to make use of social media sites to promote it. If you have existing social media accounts that already have substantial following, make use of it. You can also create an ad about it and post it to sites like Craigslist, Backpage, etc.
I know Facebook is a great tool one can be used for marketing purposes, but I am not sure if buying facebook likes is a good idea at all. From my point of view, this will only generate irrelevant likes, and will offer less business to none. Gradually gaining genuine likes is still the best way to go.