|Raspberry Pi 3 Model B/B+||$30 (either model B or B+ should work fine)|
|8GB or larger MicroSD memory card||$5 eBay|
|USB Power Adapter||$5 eBay|
|USB to serial RS-232 interface adapter||$5 eBay. Make sure it is Linux compatible.|
|Serial RS-232 null modem cable FEMALE/FEMALE||$5 eBay|
|Total project cost||~ $50|
You can also choose to use a Raspberry Pi Zero which will save you some $15 from the total price, but for the sake of simplicity, we'll be using the Raspberry Pi 3 Model B on this tutorial. I've only tested these two Raspberry Pi's, but I see no reason why Virtual Modem wouldn't work with any Raspberry Pi or even other Linux platforms.
On to the next part!