Depends what it is. Often new tech comes with compromises, and if you wait long to upgrade, the older stuff without those specific compromises become hard to come by. I couldn't tell you about your monitor specifically. It's possible that Dell cleared out its' stock of these and is no longer offering the deep discounts they often do. It's also possible the panel economy as a whole is rising in price. Either way, I would say that if it's not worth that much to you, you should just sell it. Don't keep it lying around because it might be special if you're not going to use it because you want a bigger display. That would knock a good amount off of the price of something new. If it's listing for that price, I would undercut and try to get back what you paid fast.
This review doesn't make is seem all that spectacular for anything: https://www.tftcentral.co.uk/reviews/dell_u2715h.htm
I've been wanting to get a 27" 1440p monitor for a better match as a second monitor to my 34" 3440x1440. The one I want is high for an older model: https://www.amazon.com/dp/B017EVR2VM/?c ... _lig_dp_it
so it's probably a supply/economy thing.
I'm definitely not going to actually pay that price for it. In fact, I'm not 100% sold that I need it at all. My secondary 24" 1920x1200 monitor sits further away from me, and the bigger pixels are easier to see without scaling.