rust repairs depend on how bad it is...but probably around $500-800 or so, including paint and stuff. And about your trunk, it's probably something wrong with your latch. Check to see that everything is tight. And then, slowly close it, but not all the way (I'm assuming you have a 4door since you said trunk, not hatch) and set the trunk down, but don't latch it. If it moves to one side or the other a lot when it gets to the latch, then your latch needs to be adjusted.