It's hard to find a fully libre design in your price range. Most will be based on Allwiner or similar SoCs, which are probably not vendors you want to support, regardless of the availability of opensource code, not to mention schematics. In addition you probably won't find WiFi without closed, binary blob firmware.

Having said that, there are such vendors and boards. Texas Instruments, NXP and ST Microelectronics are vendors with first party OSS support, although you might have to accept an EULA. In addition both TI and NXP have support in Linux Foundation Yocto Project. ST probably has it too but I'm unable to check right now.

Most, if not all, of this SoCs will be using PowerVR graphics, which traditionally didn't play nice with OSS, but the current free driver has performance parity with the priopretary one. This is probably a compromise you'll have to make.

On to boards themselves. First vendor which comes to mind is BeagleBoard, particularly PocketBeagle and BeagleBone Black. Second, which I believe also has schematics available, is MYiR (myirtech), although the software provided will be likely outdated, so you'll have to port or update to something more modern yourself.

Lastly, when asking for fully libre hardware, please specify where you draw the line - when you get to the bottom of it, nothing is fully open. Software is one thing. With hardware some vendors will provide schematics, rare ones will provide board design files, but there are no open source SoCs around. Similarly, most WiFi chips require closed source firmware.

It's hard to find a fully libre design in your price range. Most will be based on Allwiner or similar SoCs, which are probably not vendors you want to support, regardless of the availability of opensource code, not to mention schematics. In addition you probably won't find WiFi without closed, binary blob firmware.

Having said that, there are such vendors and boards. Texas Instruments, NXP and ST Microelectronics are vendors with first party OSS support, although you might have to accept an EULA. NXP has support in Linux Foundation Yocto Project, some TI boards are supported too (BeagleBone Black for example). ST has first-party Yocto layer on GitHub.

Most, if not all, of this SoCs will be using PowerVR graphics, which traditionally didn't play nice with OSS, but the current free driver has performance parity with the priopretary one. This is probably a compromise you'll have to make.

On to boards themselves. First vendor which comes to mind is BeagleBoard, particularly PocketBeagle and BeagleBone Black. Second, which I believe also has schematics available, is MYiR (myirtech), although the software provided will be likely outdated, so you'll have to port or update to something more modern yourself.

Lastly, when asking for fully libre hardware, please specify where you draw the line - when you get to the bottom of it, nothing is fully open. Software is one thing. With hardware some vendors will provide schematics, rare ones will provide board design files, but there are no open source SoCs around. Similarly, most WiFi chips require closed source firmware.