Fax server - looking for general info

As our overworked fax machines die off, I want to look into the option of going to a fax server. To determine if this is something I want to start now or after our new ERP implementation, I'd like to hear some feedback from those who have gone this route.
How much (roughly) can I expect to spend on a faxserver to handle 4 fax lines and about 15-18 internal people?
Any suggestions, warnings or advice is appreciated!