There's also things like the CH32V203 [1] which is a TSSOP-20 with hardware USB and costs around $0.81 in singles. The software side might be a little more ... challenging, though.
They offer some fairly usable example reference code for a lot of the on-chip functionality-- including USB peripherals. I was able to turn their CH32v3xx examples into a pretty capable custom-keyboard firmware.
https://www.wch-ic.com/products/CH32V203.html?