The Microsoft Store sells a Lenovo 28" monitor (L28u-30) for $249. This is $50 less than list price, making it the least expensive 4K display available.

Regardless of size, this is the best price I could find; sorting through the in-stock choices at Best Buy, the cheapest 4K monitor is $349.99, also a 28-inch model (Samsung UR55). Prices only go up from there, with some 27-inch monitors around $450.

Almost the same at Newegg: the Acer model is $329.99, and after that you'll be paying $350+ for a 4K display.

Lenovo's L28u-30 is less flashy, but it has an IPS panel surrounded by a thin bezel. Brightness is 300 nits, which is better than the 250 nits found on entry-level monitors, but not super-bright. Also, the refresh rate is a modest 60 Hz.

What this monitor lacks in speed and facilities (such as G-Sync and 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 (overclockable to 144Hz), G-Sync support, and DisplayHDR 400 certification, all great things, but it costs $1,199.99 Dollar. At this price, you could buy four of these monitors and still make change.