This may seem like a dodge to your question, but I firmly believe that in this day and age every entrepreneur, manager, president, or sales guy has to become a social media expert of sorts. You can farm it out - but even then someone on top of what's going on with the company really needs to drive the efforts. The good news is that it isn't that hard to learn.
There are no "Bona Fide Social Media Consultant" credentials someone can just yank out and show you. They don't get a special badge or shield once they hit a certain level. If you can't become your own experts on this field, you'd have to go about it the same way you go about hiring any help: Interviews, References and as much proven track record as possible.
To be honest, I'm not a fan of consultants until you know where the problem lies. What do you want them to achieve? Having an idea of what the specific problem is will give you more control, and more chance of them achieving your goal. (I get most my income doing consulting, albeit not in the social media sphere!)
Most important, know what you're wanting to achieve via social media; is that simply more traffic, or engaging with existing customers and dealing with after sales support?
If you're trying to generate buzz and have existing customers, give incentives to them to send invites, or post about you. Perhaps a free month of premium service for all users who a) 'Like' and b) post a review on your Facebook page. Or the best 10 get 20% off next order. Only you know the sort of incentives you're able to offer and may work with your customer base.
Remember to strike a balance here - you're trying to get happy customers to help get you referrals, not browbeat them via email until they are unhappy customers!
Do all you can to make it easy for your customers to join in.
Be responsive and react to comments about your service/products. It's likely to be somewhat time consuming though. Good luck!
I am a firm believer that you cannot be a social media expert/guru/consultant or whatever other terms people are throwing around. All a social media expert is someone that says, "Oh, you should create a Facebook or Twitter account" and then will start adding random followers to your accounts and spamming people.
Social media consultant is an umbrella term for someone who knows how to use Facebook and Twitter when in reality it's not that hard to do it yourself. We will all be social media experts one day.
So the answer is : there is no such thing as a social media consultant no matter what anyone tells you. Social media is time consuming, so that would explain why some people are somewhat successful proclaiming to be experts because companies don't have time to add followers to Twitter or get new Facebook friends themselves.
Well I have spoken to many who claim to be experts.
The ones that seemed most likely talked about realistic climbs in google ranks for their clients.
Basically the tools are there to measure them better than most consultants.
As for how much and expectation I would interview several and get a feel.
This week in startups had a good interveiw a little while ago with a guy doing crowd sourced seo.
To identify a social media "expert" look for the one that provides the most convincing details about providing metrics for success... in the sense that they can convert the use of social tools to drive traffic to your site/service/product... but also convert the traffic to users.
Gaining you a large number of fans on facebook or twitter followers is no good if it doesn't convert a percentage into your customers. So don't use followers as a metric for success.
Use the final total of new customers from your campaigns or social efforts as the only reliable metric. Typically it follows the rule of diminishing returns but you can get new users.