Everybody just about shit their pants when this was spoiled for Magic Origins. It was pre-selling at a whopping $20 apiece. It was garnering obvious comparisons to Timetwister.
Day's undoing has made hardly a splash at all in Standard, but it's seeing serious brewing in Eternal Formats, especially Modern and Legacy, where Timetwister and Wheel of Fortune are both banned.
I think the recent upswing in price is a combination of long-term speculation and Eternal demand which has been bolstered by the buzz caused by Eternal Masters. I think the market is still limited for Day's Undoing, and the near-term ceiling is likely around $5, but I suspect non-Foils to climb to $8 apiece within 2-3 years, and I don't foresee a reprint for a long time, if ever.
For every reason, Foils are your best long-term spec here. You can get these at $10 apiece right now. Next-stop is $20, and the mid-term to long-term tendency to go higher is quite pronounced.