Not counting my custom-designed waterblocks, my water-cooling system cost me about US$100.00. I purchased my parts at the 'Depot (reservoir, fittings, waterpump for fountain, hoses) and the Auto Parts store (Hayden Trans-cooler radiator and redline water wetter). The waterblocks are prototypes of a design that I am refining for possible sale, and cost about $200 each! However, you should be able to pick up a good block online from a number of sources for $30-50 with a mounting clip.
Because of the inherent advantages that watercooling affords, most systems have more thermal capacity than the load applied by the CPU. Check out the water cooling section on the front page for some great ideas on how to get started. Lastly, make sure that you have a case that is large enough for the cooling system... I have to buy a new case and PSU to accommodate my watercooling system!
Good luck and keep us posted on your results.