I'd do it like any other service, fashezee - determine the amount of time you will spend on it, and then price it by whatever formula you usually lose.
Social media marketing is a lot like link-building. Both can consume as much effort as you want to put in, and you have to set your objectives and your limits.
One difference is that Twitter is of necessity an ongoing, continuous activity. You can't "do Twitter" for a month and then relax. As soon as you stop interacting, you'll fall off the radar completely.
Hence, it's important to make the client understand that social media marketing is something like opening a retail location that has to be staffed all the time. If you are going to start tweeting, or put up a Facebook fan page, you have to keep after it - an abandoned or neglected effort is usually worse than none at all.
If the client thinks this is like a static web site - build once and forget for a few years - you'll either have to educate him or decline to get involved.