Ventrilo
http://www.ventrilo.com/
Average cost to have your own rented server actually varies a lot from company to company.
You can NOT have your own personal server in your house unless you have a several thousand slot server, and are a legitamate business...
Anyhow, cost to have a server depends on how many slots you are willing to pay for. By slots, I mean how many people connected to it at the same time...
Cost for a 25 slot server runs 12-25$ / month
These Real Time chat clients make the game more efficient, no need to stop to type, just ask away, ideal for God Runs for linkshells, and great way for say leaders of Linkshells to chat with each other in private.