Nekiwa,
We don't discuss rates here on VAF but you can get a good sense of what other VAs charge by visiting their websites. Google for
Social Media VAs and you will find many.
What you want to offer are the services you feel will help your client either get into the SM venue or help those to enhance the SM media presence they already have.
I find that these are the two main types of services that clients want.
So think about the 'big three'...LinkedIn, Twitter and FB. What can you do to get them started? Set up a FB page? Set up a Twitter page?
Do you have graphic skills? You can create Twitter backgrounds and FB landing pages.
You list the items you want to offer and you charge a package for those services. You can list add ons for extra such as maintaining posts and setting up tweets, for example.
It is my opinion that a VA (or anyone else) cannot be the entire "voice" of the client. The client must interact on the social media sites. At the very least they must also confer with the VA to advise them of what is going on in the company and so on. I have found this to be very difficult for the client to do. I've termed some clients because I wasn't really performing what I deem as social media marketing assistance.
So the best route, I believe is to offer set up services and coaching. Both of these are cut and dry and easier to package. Packaged prices are value-added for the client as opposed to hourly.
Take a good look at the social media category
here and other VA sites. Figure out what you will offer and how long it will take you to do it. I use my own FB page for example as a chance to experiment on time.
Janine