Cheap is okay. But I wouldn't base my decision on an SEO company just because they're the cheapest.
You should look up reviews online about the SEO company and ask people on forums if they have experience and what experience they have with the SEO company.
One forum you can ask such questions about an SEO search engine optimization company is http://webmasterbabble.com
Of course you can also ask about the company on this website too.
Also remember that SEO takes time and you need patience with SEO for your website to reach the top of google even with a good SEO company.
Also no SEO company can guarantee you'll be on top of google search results.
It can take 90 to 180 days to climb the google ranks on a new blog article you posted.