dd-wrt router is the cheapest but you need quite a bit of knowledge to configure a USB partition, install and configure asterisk
IP-0x is the simplest all-in-one. May wait to see if ATCOM has further sales again
Building a linux server is considered the most economic and efficient ways as it has more power than flexibility. If you do not want compilation, PIAF, FreePBX, Elastix ...etc are turnkey software. Or if you want a simpler software, you may get a simple VM image to make the configuration.