What a Server Can Do
Here are a small list of popular servers before we get to costs. If your hardware can handle it, you can run many of these simultaneously, making obsolete the need to purchase multiple servers.
Samba File or NFS Linux File Server: Acts as file storage (NAS)
Apache: Web/domain hosting
MySQL: Database server for web content
FTP: Protocol to transfer files over the network or internet.
Print Server: Enables network wide printing.
Windows Home Server: An all-in-one home solution by Microsoft to manage storage, your network, printers, and more.
Windows Small Business Server: An all-in-one business solution by Microsoft for running the network (intranet and internet) of your small to medium sized business (less than 75 users).