Bandwith Controllers

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I'm going to mount a public wireless for local institution and I need a cheaper solution for the bandwith control. They're going to share a 4 Mb. internet connection and they want to limit the bandwith (only the bandwith, not users).
A really good solution is the Ydi's Bandwith Control Unit ( http://www.ydi.com/products/bcu.php ) but it's too expensive solution.
Anyone knows any other hardware solution (a cheaper's one).
What about a "Radius Bandwith Only" server, If its possible I want a solution without username and/or password, i mean without any kind of autenticathion.

Thanks in Advance.