Do you need support for Linux? Do you need support for promiscuous mode? How big an antenna are you willing to have sticking out of the laptop or desktop? Need some more information to give good recommendations.
If you are not using windows, I would suggest an Alfa AWUS1900, but if you use windows it has horrible windows drivers (but some of the best for linux/OS X). If you need windows compatibility, the Alfa AWUS036AC is good, but isn't going to be discreet if you are using it on a laptop. If you want something a little more discreet, look at the ASUS USB-AC68 or a D-Link DWA-192 (if you can still find either of them at/near their list prices as opposed to the limited stock price gouging prices).
None of these will be under $20. The second one is probably the closest to that price range though. The problem you are running into is that wifi 5 is now in the "old enough that we start raising the prices due to scarcity" category of hardware, and these types of things, even though millions were made, were made a throw-away-able (i.e. they didn't cost enough when they were new for people to attempt to resell them used).