Haha, its not quite that simple.
Try doing a little research first:
How many people will the server support?
The hardware you want to buy for the server to support them?
Will I need to grow?
Can we use SIP or do we need BRI / PRI / PSTN interfaces?
Do we need a dedicated Internet Connection for it or can we use QoS on our existing connection?
Do we want SIP Desk phones, ATA's or Soft Phones (Please not softphones).
You are the only person who can answer those questions. From that YOU can determine yourself an approx cost to a system
Either that or look in the Elastix store for the hardware cost of a pre-packaged system