I fix computers in addition to school, but I'm learning to build speakers and stereo equipment.
41hz.com sells some excellent kits. A properly built Amp2 (85$ for a 2x 300w tripath amplifier) with a decent preamp (Alps pot + op-amp buffer ) and power supply (40$ will get you a surplus industrial transformer and some 12,000uf capacitors from apexjr.com), and you have a completed amplifier for about 150$
You could easily sell one for twice that - maybe more.