You pay 30 pounds for use of their proprietary network. I don't know how they work things out like hosts having less lag or whatever though. If it's any consolation though, the only games for the PS3 that I know of that use dedicated servers are Resistance and Warhawk.
I read somewhere that you only really need them if you have huge amounts of players playing multiplayer. For games with 8-16 people though the machines work just fine at handling the server and game at the same time.
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