// Before proceeding add an ID `target` to the table you want to perform the analysis on

// Google Chrome intercepts $ as a shortcut for document.querySelector() and
// $$() as a shortcut for document.querySelectorAll()

items = []

$$('#target tr').forEach(
  (row) => items.push(
    parseFloat(
      $$('td p font', row)[3].innerHTML.replace(/,/g, '.'))
  )
)

// remove header
items = items.slice(1)

// average it out
items.reduce((a, b) => a + b) / items.length