mirror sale in league could take weeks if not months for exalts ingame ... thats why you get "lowballed" here ... because you most likely wont sell at all ingame before league is almost done ^^ ...
getting 15k fg on a league start (which i saw a mirror getting sold) is lowball when you look at ex:mirror price at those times ... but still 2 times the fg it would be if you sell now for exalts and sell those for fg (if you sell at all) not many people out there with 180 ex that also want a mirror ^^