make your own HID by wiring them, and building your own reflector. It cost me about $30 for a 150w HPS this way. BUT, if youre looking for the cheapest light source, then youre best luck is compact floroesents (CFL for short), theyre just small floro lighting but, with a built-in ballast. They usually fit into regular house sockets and reflectors. They do a pretty good job for vegatative and satisfactory on blooming. This was my first choice when i started off, but now its all HID. If you have a bit more money then get floroesent shop lights instead (non-cfl, long tube ones). Make sure you use both warm and cool lights for best results. Floros can be kept inches from the plant tops too, unlike HIDs which has to be kept about a foot or two.

DO NOT use halogen or incandesents for a cheap alternative, they release most of their energy through heat instead of light and are the wrong frequency for plant growth.