A used one from eBay goes for as little as $40. Businesses are dumping their old thin clients by the ton, so you can find them quite easy for cheap. The biggest problem with them is that it’s never all that clear what exact configuration you’ll get (e.g. some CPUs might be more power hungry than others).
They are quite a bit bigger than the N3350-based MiniPCs, but depending on what upgradability and ports you need, they can make a good alternative. It’s after all kind of the fun with the mini PC space, there is a ton of stuff with different configuration and price ranges. And unlike the SBC space, it’s all just plain old PC that you can boot stock Linux or Windows on, no need for special purpose RasbianOS and the like as in the ARM world.