How much is a backlink worth?

It depends on:

·        Website (domain) authority (Trust Flow, DA, Ahrefs domain rank are more or less good metrics)

·        Page authority (Real time Pagerank, which unfortunately you cannot check)

·        Relevance (is it in your niche?)

·        Anchor text (Both keyword and Brand links have their advantages)

·        Placement (Header/content better than sidebar better than footer)

·        Dofollow/Nofollow (Nofollow gives almost no value)

·        Number of external links on the page (The fewer, the better for link juice, but Trust flows to you no matter the number of links)

·        Geotargeting (is it from the local Google you want to rank for if different than Google . com)

Can be from cents to hundreds of $ per month. You should always remember that links have ongoing value for your rankings.