We used to OEM HP ProLiant DLs at my last gig. Great hardware. The new DL320e starts at $579 list, so will definitely fit within your price point:
http://www8.hp.com/us/en/products/proliant-servers/product-d...
If you plan to buy a few of these, definitely give their OEM / enterprise sales guys a call - they give very good discounts off list (based on volume, of course).