Affordable SEO is one of those services that’s kind of like a holy grail: lots of people pursue it, but most of them (and lots of people that don’t bother) think it doesn’t actually exist. That’s because of almost anything you could buy in this world, SEO seems set up to cause buyer’s remorse. You pay for it, you don’t really see much — maybe a summary of activity from your SEO company — and when the results do come, there’s nothing really concrete that you can attach them to.
Would you have ranked without that last month of SEO? Was it really necessary? If you don’t pay for it next month, will you slip off the first page? The entire world of SEO is nebulous and abstract to a webmaster, and that makes it very hard to know what SEO services are ‘affordable’.
The succinct answer is this: if you can make more at the end of the year by paying for SEO services than you would have without them, they were affordable. Ironically, that means that two more-or-less identical people can purchase the same SEO services at the same price — and it will be affordable for one guy and not the other. The first guy will buy the SEO, work his butt off making killer content for his site, and monetize it to hell and back. The second guy might miss out on one or both of those steps, and end the year making less than he spent on his website’s SEO.
In the end, it’s really up to you. Do the research and get the best price you can for a company to build you a natural link profile and do your on-site SEO. Then, commit to your website’s success, and push it. The more you put into your site, the more you will make from the traffic that the SEO company drives your way.
To paraphrase an old maxim: laugh maniacally with the rush of victory, and the world laughs with you. Sulk in the corner because you didn’t crush it, and you sulk alone.