The cheapest is to use the 6.5K bulbs for mercury ballasts. It is possible to get 250Watts for under $100 for ballast (~$25), bulb (~$60), and socket($5).
If you want to use 10K, 14K, or 20K bulbs, it gets harder to find a "cheap" solution. http://www.thelogicalreef.com has 175 watt 10K...