It's mainly the laser itself that is the expensive part. If you only care about resolution it's easy, you just need a single-mode laser. But if you care about accuracy it's very difficult, because then the wavelength needs to be stable, and that requires a much more expensive laser. Most people looking for an interferometer are interested in accuracy, unless they're just measuring vibrations.
You can get pretty far with cheap diodes + current and temperature control. Unless you need coherence lengths in the meters range you can make do with cheaper lasers.
