How Valuable Are Your Site's Backlinks?

As one Redditor claims, links are worth "pie." There is no real right answer. A link's worth is highly determined by a page's rankings on Google's SERPs (Search Engine Results Pages) and the amount as well as the type of traffic the page draws in.
When link building, it is always a good practice to consider pages with high page-authority (PA) and pages which are highly relevant to a given niche. However, page-authority and domain-authority (DA) are not reliable factors to determine a link's ranking, and therefore, worth. Both terms were invented by the SEO gurus over at Moz and should not be used as the basis for any SEO decisions. DA and PA are known to be inaccurate. Moreover, the link database utilized by Moz (Opensite Explorer) is notoriously unreliable.
It all depends on a page's ranking, keyword relevancy and the type of traffic desired as a result of a successful backlink. Certain domain metrics are not enough to calculate the worth of a backlink. If a particular blog or page is highly relevant in a given niche and ranks well in SERPs, it can still be a beneficial backlink. Furthermore, all claimed traffic should, obviously, be verified before coming to a decision on a link's real worth.
Everyone buys and sells backlinks at different prices, all with promises of SEO grandeur and quality traffic. There has never been a general, set price to pay for quality links. People set their own prices based on a page or blog's relevancy within a given niche and the links potential worth to a viable buyer. SEO experts really have to rely on their own judgment when shopping around online for quality backlinks.
In conclusion, all of this is entirely irrelevant. Most people promoting backlinks for sale are up to no good. There are millions of scams online from supposed "quality" pages and blogs. The general rule of thumb for SEO linking should be: never pay money for a backlink.