Q: What's the overall cost of this thing?

A: A quick back-of-the-envelope estimate would be:

• laser: 250€
• microscope objective: 150€
• diffraction grating: 125€
• filter+DIC-mirror: 75€
• OAP-mirrors: 350€
• elliptical mirrors: 150€
• lens tubes, fiber adapters etc: 100€
• fiber optic: 100€
• cases: 100€
• linear CCD module: 25€
• connectors: 25€
• misc electronics: 100€

So all in all around 1500€. It can certainly be made cheaper, but now you have an idea.

Q: What's with the expensive laser?

A: If you want an instrument capable of making reproducible measurements, you need a stable monochromatic light source. Cheap lasers are neither. However, with patience you can find used JDSU and Coherent lasers for somewhat cheap (100-300$).
https://erossel.wordpress.com/2015/07/26/clear-as-the-mississippi/

Q: What's with the not so cheap microscope objective?

A: The intensity of the Raman signal is roughly proportional to the numerical aperture squared, so you want as high NA as possible. You can get cheap microscope objectives with NA > 1, but they are oil immersion with a working distance not much longer than standard cover glass thickness (0.17mm). The OEM version of the Nikon CFI VC 20x NA 0.75 was the cheapest high NA objective I could find with a long working distance (if you consider 1mm to be long) - and it's made for fluorescence applications so there won't be any autofluorescence.

Q: What's with the expensive edge filter and dichroic mirror?

A: You need efficient blocking of the Raleigh-scattered light, so you need proper filters. And they weren't that expensive - I bought production overruns from Omega.

Q: Why don't you have a slit in your spectrograph?

A: A slit is a waste of light if you cannot focus the entire length of the slit onto the linear CCD, and for that you need a cylindrical lens. Both the slit and the lens are expensive, so I chose to simply use the optic fiber aperture as the point source.

Q: What's with the custom fiber optic patch cable?

A: The raman signal is weak. I wanted a fiber optic that wouldn't pick up light pollution. It was 30€ extra - but of course around 80€ more than a lucky *bay find.

Q: Why the super expensive mirrors?

A: The off-axis parabolic are just not cheap, and once you get into a serious collection of mirrors, the tendency is to push it as far as possible.