Using an off-the-shelf mouse sensor comes with a challenge: Since I'm flipping the sensor upside down, then rolling a ball on top of it, the X axis is inverted. Not ideal for a daily-driver trackball. I also want the ball to be able to be used in different orientations, and with buttons in a separate module.

Software axis remapping programs and drivers exist, but are finicky, especially on certain platforms.

Instead, I opted to try jfedor2's HID Remapper with a Waveshare RP2040-Zero, clones of which can be acquired for < $2 on AliExpress.

Despite my poor soldering skills, it worked, and I was able to remap and invert the X axis without issue.

That should solve the hardware cost problem. Replaces one ~$15 component with two $2 components.

Now to figure out how to add buttons, then design an enclosure that allow reorienting the trackball and separating out the buttons to allow usage with split keyboards.