The recently announced Raspberry Pi 4 is said by the Raspberry Pi Foundation to be a "desktop replacement"; this is disputed by some, but if you just mean simple stuff, email, web browsing, simple office stuff (Libre Office) then these are perfectly possible. It comes in 1 GB, 2GB, and 4GB RAM sizes, the ones above 1 GB are more expensive. Remember the price of a Raspberry Pi is just for the board and does not include a case (around $10), a power supply (you need a 5v 3.5A one, not any old phone charger, the official one is $10), nor a keyboard, mouse, or monitor. It has built-in wifi, Gigabit Ethernet, and two USB 3 ports (and 2 USB 2 ones). If you can find a case with a VESA mount, and you have a suitable monitor, you can make a neat little all-in-one system.