The best deal around for a 4K monitor can be found at the Microsoft Store, which is selling Lenovo's 28-inch model (L28u-30) for $249 right now (opens in new tab). That's $50 below its list price, and the least expensive around for a 4K display.
It's the best price I can find regardless of the size. Sorting through the in-stock options at Best Buy, the cheapest 4K monitor costs $349.99 (opens in new tab), which is also a 28-inch model (Samsung UR55). Pricing only goes up from there, with even some 27-inch monitors commanding around $450.
Lenovo 28-inch 4K (3860x2140) Monitor | IPS | $249 (save $50.99) (opens in new tab)
This one is not brimming with bells and whistles, but the price is unmatched with this deal. At $249, this is the least expensive 4K display around that is (A) new and (B) in stock.
Lenovo's L28u-30 isn't real fancy, but it does sport an IPS panel, surrounded by a thin bezel. Brightness checks in at 300 nits—not super bright, though above the 250 nits found on some entry level monitors. It also has a modest 60Hz refresh rate.
What this monitor lacks in speed and amenities (like G-Sync or FreeSync), it makes up for in price. For example, Acer's Predator XB3 is a 27-inch 4K monitor with a native 120Hz refresh rate (can be overclocked to 144Hz), G-Sync support, and DisplayHDR 400 certification—all nice to have, but it costs $1,199.99 (opens in new tab). For that price you could buy four of these monitors and still have change left over.