Price open-jaw tickets vs. single destination to see where you come out, but in my case, flying in to Paris and out of Frankfurt was priced appropriately (i.e. no extra cost). Sometimes, when it is $500 or $600 more, it's not necessarily worth it, so research!
I am dropping my car (picking up AFTER Paris, to do French countryside) in Metz and spending the night before picking another up in Saarbruecken to avoid the dropping off in a different-country fees. I will take the train from Metz there, but I like the idea of just staying with the train once in Germany and doing the Rhine tours. I have a different situation (will be spending another 10 days in somewhat rural Germany, hanging with relatives), so I need a car.