Cheap SEO services are pretty much all scams. There is no effective SEO that can be done cheaply. Google closed most of the previously-effective loopholes in the past year. Using some of those old strategies are extremely risky now, and you can get massively penalized. What the cheap SEO companies do is spend about 1 hour a month doing some rudimentary shit that is essentially worthless. If it does work, it's probably shady stuff that will end up getting you penalized to the point that your site becomes useless.
Most firms charging under $500-1000 a month are scammers.