There are many metrics but these days the most commonly used metric to find out the value of a website is Domain Authority.
Domain Authority tells how well your blog or website can perform in search
Domain Authority (DA) is a search engine ranking score developed by Moz that
predicts how well a website will rank on search engine result pages (SERPs). A
Domain Authority score ranges from one to 100, with higher scores corresponding to a greater ability to rank.
Domain Authority is calculated by evaluating multiple factors, including linking root domains and number of total links, into a single DA score. This score can then be used when comparing websites or tracking the “ranking strength” of a website over time.