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.

Only part of the project that looks expensive is a neon sign transformer. Other than that, the hardware is pretty cheap. For the gas, CO2 and helium are easy to get, but I need a pure source of nitrogen. It may be enough to burn something in an enclosed space, so long as I could get the smoke particles out. That would leave CO2 and Nitrogen.