Prices will vary considerably based on what you want accomplished; the breadth and scope of the project undertaking, people involved, whether it's ongoing (which is always desirable as it garners more results, hence more returns on investment), or a single "one off" effort, if there's training needed, ongoing maintenance required, and actually more than that. And prices will also vary depending upon level of professionalism and overall quality: a professional with 15 years' experience can (and in my opinion, should) charge more than someone who's just starting out and may be inexperienced.

One way to estimate cost is to work out in specific detail what you would like back in return for your investment. So if you stated you want more traffic to a site, I'd ask you what type of traffic do you want? Do you want stay-at-home mothers to patronize your site or corporate attorneys, chefs or CPAs? Any professional with real-world SEO experience can be as specific as you may want. The less targeted your goal, the less targeted your results will be and more scattered...and the less legitimate leads you'll get from a campaign.

What's your budget for a) hiring the SEO/SEM/Social Media expert and b) what's your budget for paying for that professional's budget? This is something non-developers or business owners new to SEO and internet marketing seldom consider. Or is it all one lump sum? Advertising on Facebook can vary from Google Ads, Bing will probably be cheaper but deliver different results from Yahoo, and so on. And all of those advertisers charge different rates and use different metrics to measure and quantify results.

So, without getting too "techie" (if I haven't already done so), I would budget (at least) what I would to place an ad in a local newspaper for several months or what it would cost to place an ad in a local area weekly, or what you'd expect to pay to join a local Chamber of Commerce - with the caveat that a professional web developer with a background in SEO/SEM and social should be able to deliver great return on investment and in more varied ways across the board So whomever you'd work with, you'd want to make sure they can inform and/or train you enough to be able to value and understand what they're doing and how their delivering value to you, make sure they understand branding, content marketing, have testimonials and professional memberships, use contracts, and ask lots of questions. You'd want to build an ongoing partnership with the person to get the best results.

Hope that helps. Let me know if you'd like more details on anything touched on.