There are a few ways to go about it.
DIY
ASRock h81M HDS + G3240 ~ $148
Crucial 4gb 1600mhz Value Ram ~ $52
WD blue 1tb ~ $68
Inwin BK644 + 300w PSU ~ $109
LG DVD Drive ~ $21
Windows 7 Home Premium 64bit OEM ~ $115
Total ~ $513
Optional Monitor:
Dell S2340L 23inch eIPS Panel ~ $198
Dell U2414H 24inch eIPS Panel ~ $340
Or alternatively, you can buy a barebone PC suce as intel NUC, MSI Wind, Gigabyte Brix, Zotac Box etc.
Customize according to your needs should not exceed $550