$5.5m for Finnegan doesn't seem that bad considering the pretty ludicrous CB/S market this offseason.
If he plays like he did last year, the Dolphins would be better off starting whatever UDFA cornerback they can find. If I'm not mistaken, Finnegan gave up a perfect passer rating to opposing quarterbacks both in the slot and on the outside before he was placed on injured reserve last season. Why would any team pay $5.5M guaranteed for that?