It is indeed possible to correctly optimise a website yourself to get to page one of Google quite easily, in fact I am quite good at it even if I do say so myslelf, its not an exact science but you need a deep understanding of its algorithm and if you dont have time to devote to it you may as well pay a specialist to do it for you.

That said, I still pay google £5000 a month on average as my organic hits simply are not enough for my industry.