Mostly 3D printed Michelson interferometer - all the structural elements are 3d printed, anyway.
OpenSCAD files: github.com/Erhannis/OpticsBench
Other components:
20x20x1mm mirrors: $2.54 www.aliexpress.com/item/32842639748.html
2'x4' pegboard: $9.21 Home Depot
Laser Pointer: like $2 from anywhere
50/50 Beamsplitter: ~$20 on ebay
Total: ~$34 (plus 3d printer and some filament)

Sorry about all the other junk in the space; only some of the visible objects are necessary.
I might do a video on aligning the thing, eventually.