To me, if you're not spending money and not making a ton on the AH, then the question is not so much about how much more those three will cost you and more about whether or not the value of what you have right now will go up, or down, in the near future.
I guess people are saying that value goes up at least temporarily as the set becomes completable. If so, then I would advise you to sell off anything valuable as their value spikes.
I don't think Mikan or any of the Amethyst Historic Rewards are worth the value of their collections. At least not for more than another month or so.