What has helped me a great deal in creating social media strategies for clients is when I get questions like 'We want 2000 twitter followers' or 'We want 1000 fans', I ask them very basic questions like, 'What are you hoping will result from having 1000 fans?' Questions like that get them thinking strategically. I always throw it out there that if their looking to get to a certain number of fans, there's sites out there like fiverr that allow you to buy thousands of fans or twitter followers that are mostly bots - and that they don't need me to do that.
Having that conversation will get the client focusing on the proper long term strategy.
In my experience you really have to get the client out of caveman mode ("ME WANT MORE FANS!") and once you do that you can position social media services with the proper expectations.
Obviously you know that the costs for fans will be all over the place depending on your CPC. In my experience for traffic I send to the clients FB Page (most of this data is pre-timeline though) the rate of clicks to 'fan conversion' is in the 15% neighborhood. So once you identify your CPC, you can use that number to give a ball park idea of what a cost per fan acquisition is going to look like.
If they balk, you can share articles like this:
Understanding the Value of a Facebook Fan
Which will help you show your clients how facebook and twitter can be a platform to drive conversions rather than simply a way of collecting fans and followers.