What CPU do you plan on using? If you want the cheapest and lowest power usage (wattage not GHz), you can get anything from the $100 AMD E-350
which has a low power (wattage) CPU @ 1.6 GHz built into it up to a low end AM3 or Intel 1155 (H67 or H77) build. There are many to choose from, depending on how you plan to use your PC. Home theater, budget gaming, just for surfing, best all around? Please elaborate.