It's unfortunate that cellular providers don't offer this. I get around it by setting my data connection to prefer 3g. It helps with a 500mb limit (data is expensive on cheap plans), but it'd be nice to be able to throttle apps individually (no, I do not want video ads to be downloaded, but internet connectivity is required for the app to work online. A HN client with webview, for example)
years ago i came across per gb pricing but the cost was insane. it was essentially: prepay your usage or we charge you 10 times as much as the prepay would have cost you.
