Allow me.
Keyword research costs. Press releases cost. Graphics cost. Content costs. Pad submissions cost. Tools cost. Citations cost. Monitoring websites and analyzing their competitors costs. In time or money.
A quality service will certainly be better at weathering the changes that come and go. Fly by nighters work cheap and promise the moon, and go get fiverr blasts. Quality providers are more expensive, unless you find a "guy" or "girl" to freelance for you.
Actually it may take months to rank for competitive keywords.
Graphics creation, keyword research, market research, social media integration, onpage SEO skills, back link building, citation building, content creation, reputation management, copyrighting, design services, functionality testing, for a start.
It's a good deal harder than that because everything is so fluid you must constantly test and adjust. A language you learn once, and you apply it. SEO you learn gradually, and live a life of tweak and test. Then Google changes shit around and you have to test and adjust what you just finished testing and adjusting even more.
Parts of it are simple, parts of it are hard, low competition is simple, high competition is hard, low traffic is easy, high traffic is hard. High targeted traffic is a blue-eyed bitch. Simple content creation is easy, quality SEO articles with headings, focus, and LSI's are hard. Quality back linking is hard, spamming backlinks is easy.
Oh fuck yeah, many times over, if you do it right.....