Watching the MTGPrice Arbitration Tool that's available for free isn't exactly a foolproof way to make money. But what it does show you is cards that certain vendors are willing to pay above the going rate to obtain. Since about April, I’ve been closely watching Mizzix's Mastery. Many vendors have been selling copies for $2.49. At one point, StrikeZone Online was buying them for over $3. MTGPrice even had the fair trade value of this card pushing $4. So it seemed that TCGPlayer and some other venues are underestimating this card's value. Since then, the fair value has come down to under $3, and some copies are even available for $2.
The supply on this card isn't exactly huge. This seems like a card that could easily be $5, especially since there has been a Mizzix’s Mastery Combo deck in Legacy that saw some limited Top 8 success. Today, the buylists are paying anywhere from $0.50 - $1.50 for it. I would definitely watch this card carefully, though. Shadows Over Innistrad introduced many interesting spells that could do well in Spellslinger decks in EDH. Granted, this won't do much good for Madness spells, since they get exiled. But Mizzix of the Izmagnus is a good Commander, so I expect this card has more incoming demand than vendors expect. I could be wrong, but picking these up at $2 or less still doesn't seem like a bad plan.
Unless the Legacy combo deck takes hold in the format, most people won’t really want 4 copies of this card. So it won't spike like some other cards have in the near future. It will take time, especially with most of the demand coming from EDH. I just don't see many of these entering the market at the current buy list rates, though. Also, Star City Games still lists them at $4.99 a piece. So there is real potential for this card to be $5 sooner or later throughout the Magic market.