For regular STM32F103C8T6 blue pills, of course there are lots of sellers on AliExpress that go lower. The lowest right now [1] is listing them for $1.53.
That said, it's really confusing how much flash memory are on these. The community seems to find that they list as 64 KB, but in reality ST has been shipping the part with 128 KB without saying. I have two black pills (slightly different board, same MCU) but the one I tested only has 64 KB. :(
1: https://www.aliexpress.com/item/32839140960.html?productId=3...
$3.49 is great, nice find. Hopefully now that Espressif has unveiled their fork of LLVM you (as well as myself :P) will be able to use Rust on the ESP chips soon enough.