Check out this open source design for a spectrometer built out of Legos. You can use their designs to 3D print your own Legos and follow the online assembly instructions (or forge your own path!). Or you can buy a kit starting at $30 and running up to $85 for a kit with a Raspberry Pi and associated camera. At the very least these spectrometers are supremely affordable. They also seem to do the job. And, perhaps most importantly, look pretty darn fun to build: 
