You can buy good SEO, but not great SEO.
This is simply because when anyone has the resources to offer great SEO, they don't need your money, because they've already got rich by doing it for themselves.
Indeed, the best SEO help comes for free. But bad help also comes free, so watch out!
What about job satisfaction?
Job satisfaction either outweighs the need for money or not. If job satisfaction is greater than money, then I would do it for free. Even if they pay me, they have not bought my time, they have merely rewarded it, and my work is still intentionally for free.
If the need to be paid for the job outweighs the job satisfaction, however, then I'd probably dust off the job ASAP, issue the invoice, and move on to the next client to make the next money; unless the first client had already paid enough for me to comfortably retire on, which is never a certainty in a world of inflating currencies.
Short-term job security VS long-term credibility
If I really want to try hard to maximise the SEO on a client's site, because I value the client's projects so much, then I would not need money for it.
If I would not do it for free, then I would not be as keen to do a good job as someone who would do it for free. I would have alterior motives. I would be more keen to act like I'm doing a good job, for short-term job security; rather than actually being keen to do a good job, for permanent recommendability.
This is why popular, free, open-source software (including web programming languages like PHP and browsers like Google Chrome) will become more and more globally trusted; whereas expensive, proprietary goods will join the dinosaurs. Computers, the internet & the world wide web are fairly new; so don't cry for proof.
Even crowded human societies are new, compared to the millions of years in which our ancestors have walked the earth. Studying will go and knowledge will be forgotten. Law will go and rights will be forgotten. Skills will go and profit will be forgotten. Time will tell.
