Have you searched on the forums. There's a topic about this a few times a month.
I've tried quite a few: GigiaNews, 100%, Easy News, Usenetserver and a few I don't remember. Easy news is great because you don't have to download any headers but it's expensive. Usenetserver is unlimited but they tend to have network and completion problems (at least when I was a member). Giganews has great retention and completion and their speed is awesome.
You'll pay anywhere from $10-$19 per month depending on the service. Astraweb offers some one time only payment options where you can buy 25GB for $10 and use it whenever. I may try that just to see how the service is. Wish they took paypal.