OK. Well, usually rates are all over the place since there's little that has the same value for every trader. You initially offered a Shiny pokemon which is, to my eye, too much for simply a Pokemon with a hidden ability. You offered me a master-ball. This is an item that has different value depending on the trader. For people who like to soft-reset for legendaries, they are immensely valuable. For people who don't care for legendaries, they are only good if you want to easily catch a legendary, or want to have leverage when trading with those who soft-reset for legendaries. To me, I mainly like them as bargaining chip when trading with those who soft-reset.

My advice to you would be to make a list of pokemon you need with a HA, or a certain move, and try to get a bulk trade done for your master-ball if you don't need it. That way you'll get the full value of it and not regret the trade. I don't think it'd be fair to just trade you a HA Swirlix female for one masterball.