Given that the Pi A+ costs about £17, 2 B £37, camera £20, my Scottish ancestry would baulk at paying more than about £20. My Welsh ancestry might go a bit higher, and the Devonians would probably max out in the £30-£35 range (but they were rich enough to own a fleet of cargo boats)
How much do you think you'll be wanting to sell it for?
... resolution of at least (589.592-588.995)/2 = 0.2985nm
So a factor of about 40.
I vaguely remember at an OU summer-school making a standard spectrometer, then comparing it to a professional one. The difference was phenomenal.
This one should still be good enough to see line spectra in a mercury source, or salts in a bunsen flame, I would think?
The other thought I had was whether it would see Fraunhoffer lines in the sun's spectrum.