depending on your needs a server could cost as little as $200 (less if you use used parts) or as much as $15000. I've personally got an Athlon 64 3200+ with 1GB of ram and 1TB (500GBx2) HDDs. all the hardware excludeing the tape drive was less than $1000 but the tape drive (260 GB sony AIT3) cost about $1000 and the operateing system (Red Hat Enterprise Linux) cost in excess of $1000 which brought the total cost to slightly over $3000(it was actually closer to $4000 when I built it but I converted the prices down for hardware price drops). If you don't need tape backup or you settle for an older lower capacity model (20GB Taravan Drives can be had for less than $300 and AIT1,DDS4,DAT 72 drives run about $500) and you use a free linux server OS or a cheeper OS such as windows server, Xandros, or a cheeper version of redhat. You should be able to build a good server for $500-$1500.
For a true network server(that has the network connections of all systems in the buisness running through it) I would recomend nothing less than an Athlon 64 or Pentium 4. For a dedicated file server (for storeing files only and not for network operations) a Pentium MMX will be sufficent (Pentium 3 or higher recomended).