Buying backlinks is much cheaper than Google Adwords. Obviously with paid links, you will pay more for buying high quality backlinks than for a link of low DA and PA value. This is normal because the higher the DA and PA value of the website, the more value the many different paid links will have.
Under no circumstances should you consider paying for low quality links or black hat practices, only one way is to buy links with high authority, otherwise you will pay for a link with no value that is and you will not rank higher in google search engine so, buy quality backlinks !