I had seen before major changes on Delta's website with pricing, but I was truly surprised by a recent experience. I booked perhaps two months ago a trip in October for 85,000 SkyMiles. Yesterday, the price was less than 50,000 SkyMiles. Today, incidentally, the price is 58,000 SkyMiles. Fortunately, I looked yesterday, was able to cancel the old old reservation, and then was able to book a new one. I had seen in the past on the Delta website major fluctuations in pricing from one day to another, which is the reason after the initial booking I often check the pricing. But I don't remember before seeing a 44 percent reduction in SkyMiles charged (and there was a commensurate reduction if one were paying in dollars).
Again, using SkyMiles, I have the flexibility of booking and later cancelling and rebooking. But I would suggest that if a Delta fare seems high, try waiting and checking repeatedly over the next few weeks for a more reasonable fare. And for those using SkyMiles or having otherwise the luxury of cancelling without penalty, do every so often check the pricing to see if the cost has dramatically been lowered.