I did this recently, costs/specs:

Case + Mobo + Heatsinks + PSU: $740

2x E5-2670v1 (8c/16t ea, not ES): $120

128GB DDR3 Ram: $240

6x 4TB 2.5in Disks: $650

Total cost: $1750

Motherboard has 2x 10GbE, 16x port LSI SAS/SATA controller. Case is a 2U, 24x 2.5in bay, with 2x 920W Platinum quiet PSUs. With a larger budget 256GB of ram and 2x 14c/28t CPUs are pretty reasonable (~$1000 more)