Sceptre 27" 4K monitor for $200 on Amazon. It's the cheapest 4K monitor I've found so far.
I wasn't even looking for a 4K monitor when I bought it, and didn't think it would make a big difference, but I was very wrong. It's made a huge difference in my day-to-day computing experience.
Reading ebooks and papers on screen is actually bearable now; previously I'd prefer to print documents out or purchase/borrow physical books for reading.
I can also now use antialiased vector fonts for programming, though I still use Terminus in the terminal emulator.