A place where I write on search marketing
Ask these companies for references. Does the company have clients who are very satisfied with their services? Are their clients satisfied with the services? Do not get caught up in trial and error, which can be expensive. Google the company, too. Google the name of the company, plus the word "forum" or "message board" to find online communities critiquing the affordable seo services you have found. Although people on the internet tend to post more complaints than praises, if it is truly a great, quality SEO company, then people should be excited about it.