The answer is equally simple: It depends. Different public relations agencies will charge different fees, so costs will vary depending on the type of publicist you decide to hire.
As an independent public relations specialist, I work one-on-one with clients directly and usually only handle one or two clients at a time. I like to meet with clients to go over their requirements, and the extent of my services is determined by what they want in the way of publicity. Some just want a press release and/or limited media coverage for a one-day event; others want speaking engagements, a book tour, a blog tour, radio and TV coverage, etc. I charge $50 per hour, and usually work about ten-fifteen hours per week for each of my clients. I've had clients who contract for as little as five hours of work, and others who I work with for several months. I keep detailed time sheets that I send out every two weeks, and I always do only what clients have contracted with me to do. Most of my authors are self-published, although I have a couple who have published with larger houses and want to do a little more than what's covered in their publisher's publicity contract.
Many writers also ask about success rates for promoting self-published books. I've been pretty successful with self-published authors. For example, one of my clients, Paul Woodring, an African American writer, made the bestseller list at HueMan Bookstore in New York for three months after reading there. It was very difficult to get him in to the store, but I persisted and finally got the managers to say yes. He's also done a featured signing at Marcus Books in San Francisco (another prestigious black bookstore that was difficult to get in to) and did a national book tour that was very successful. He's exhibiting at BEA this coming week and has had a book trailer created by an established filmmaker, so he's really put a lot of effort into his marketing.
But, even those who can't afford to exhibit at book expos and create film trailers can still do a lot with a little publicity. I recommend that most authors at least have a professional press release done, and do some book signings, even if they're local. In addition to creating buzz and making personal contacts with readers, the writer can get some additional mileage out of those events by getting his/her books stocked in the store and placing photos from the signing on his/her website. I also think a blog tour is a relatively easy way to get noticed, although it can be time-consuming to set up (I research appropriate bloggers and their sites, and then contact them directly regarding participation in the tour).
But, even those who can't afford to exhibit at book expos and create film trailers can still do a lot with a little publicity. I recommend that most authors at least have a professional press release done, and do some book signings, even if they're local. In addition to creating buzz and making personal contacts with readers, the writer can get some additional mileage out of those events by getting his/her books stocked in the store and placing photos from the signing on his/her website. I also think a blog tour is a relatively easy way to get noticed, although it can be time-consuming to set up (I research appropriate bloggers and their sites, and then contact them directly regarding participation in the tour).
The main thing I bring to the table is the phone work – I really push media and bookstore representatives to look at my clients. And I think that objectivity is what convinces them to say yes. It's hard for an author to call a radio producer or bookstore owner to pitch his own book, but when I call and say a client's work is great, the producers and store managers listen. And I'm persistent, which is key. It's often easy for people to say no, so I keep going back with new angles and approaches until I get them to say yes (this can be the time-consuming part, but it works!).
When you're ready to hire a publicist, be sure to check out a lot of agencies and individual consultants and find one with the right attitude and fit for you and your work. And don't be afraid to ask for references – a good public relations professional should always be willing to put you in touch with his/her clients so you can hear firsthand what they have to say about the publicist's professionalism, follow-through, and success rates.