In the core rules, one of the chief values of the Mentor background is training. Here, it's more akin to a source of favors and connections. The value of a mentor to whomever has them as a mentor should be about equal to the value of an artifact of equal rating. Note that this subjective value can vary depending on who that mentor is and who has them as a mentor, and the same mentor can be worth different ratings in the background depending on how many favors they do or don't provide. See an ST to work out what the value of your Mentor is specifically.