Well it depends on how heavily it is being used. And I did give the first recommendation as to go to the vendors for quotes on the usage.
It will still most likely end up costing 1500-3000 from them.
And obviously you can go lower with the CPU, Mobo and Ram if so desired in the self built system.
Generally though would be nice if the system didn't choke too badly if 50 people do backups etc at the same time.