Laser

1w blue laser diodes are fairly cheap. The driver circuits are pretty cheap, or I could build one. Need a heatsink. Some good modules for ~$10. I could probably put the whole thing together for $40.

CO2

I'd love to build one of these someday. Not easy, but no single part is too hard.

Looking around, copper looks like a good choice for the mirror. I saw instructions for hand polishing a copper mirror, and giving it a slight concave.