I don't know if it works on Delta, but I had something like this happen with our last United flight (tickets doubled in price while I was debating different dates). I just pressed the back button and was able to book my last search even though I verified on another browser first that they had increased the ticket prices for those days and actually the entire month for everyone else. I was stuck with the previously searched specific dates and didn't get the extra day that I wanted, but I saved 50% and was content with that.