How much is a backlink worth?

It is dependent on

The authority of a website (domain) (Trust Flow, DA, and Ahrefs domain rank are more or less good metrics)

The authority of a page (Real-time Pagerank, which you cannot check)

Relevance (does it fall within your niche?)

The text that appears as an anchor (both keyword and brand links have their advantages)

Placement (header/content is preferable to a sidebar, which is preferable to a footer)

Dofollow/Nofollow is a term that refers to whether or not a website should be (Nofollow gives almost no value).

The page's total number of external links (the fewer, the better for link juice, but trust flows to you no matter the number of links)

Geotargeting is a method of directing traffic to a certain location (it is from the local Google you want to rank for if it is different than Google.com).

It might range from a few pennies to hundreds of dollars per month. Always keep in mind that links have a long-term impact on your rankings.