Miller has 76 career saves and has blown just 7 save opportunities in his career. In 202 career innings, Miller has allowed 54 earned runs.
Megill was an All-Star last year and blew 6 saves that season. Even omitting his poor career numbers before Milwaukee, Megill has pitched 167 innings with the Brewers, and has allowed 54 earned runs.
That is the delta you mention and it’s significant. What it’s reasonably worth in a trade, who knows?
It is certainly a strategy to ease the SP’s burden knowing that Ashby, Uribe, Megill, and Miller are out in the pen.