Prices of the cards I am interested in are stable, and not crashing. May 21 and we are a week away from online release.
May 25 update: a 24 pack box of paper MM15 can have packaging errors or a too-good or too-bad distribution of money cards. I am glad I am not opening any paper MM15. The comments I read on the forums are a mix of very happy ones for those who got a 24 pack box with multiple mythics (someone pulling a Goyf and a foil Goyf would be very very happy), some in the middle with maybe a few nice rares and one mythic, and then the not so good boxes with no mythics and a bunch of the cheaper rares...
There have also been reports of boxes either with a mythic per pack, or with an uncommon in the rare slot per pack. There is a lot of talk on the forums about people not getting good value out of $10 packs but I think there is a lot of noise in the forums, and I mean statistical noise, and even bias since most people buy paper packs and never post anything on the forums. The tendency I think is to get reports for the extremes of good and bad pulls, and also of packaging errors.
Link to my comment about $10 packs back in February of 2015.
May 29 update:
I am definitely not opening any $7 packs for this set online.
Just kidding! I love opening packs. The EV for these packs is terrible, but guess what Wizards? Magic is my hobby, and not an investment. I don't care much about EV when it comes to cracking packs from a super-value set like MM15.
Just like with Tempest Remastered, I will keep track of my buy list. Hurkyl's Recall price is great news for Vintage.
Link to Part 10, in which I buy a bunch of cards to play this set!