With that budget, you can get almost any monitor you want, doesn't need to be fancy since it'll only be a secondary monitor
However, you can get a 32 inch 144 hz 4k res monitor in the $300-$400 range if you want something more than just a side monitor
Found one with the same specs as above for $225, and a handful of 27 inch 144 hz 4k res in the $200-$300 range
I would personally choose something that would sit well side by side with your main screen, or get 2 of the same decent monitors if you don't have or plan to get an ultrawide
You should wait if you haven't purchased one yet, and keep an eye out for sales next week
might have some good deals coming up