I'm not sure i understand your scenario: germany arguably has a more powerful economy than france, so DM would have a lower exchange rate towards euro than FF towards euro. So imho things
in france would actually be cheaper (as seen from germany). Anyway, for that we have customs (everyone has some sort of customs, this doesn't mean strong borders for people). And more generally i don't want any "US of europe": the ratio between in-country similarity and cross-country similarity is far to high across europe to let down economic sovereignty (different demographics, different cultures, different economies, i mean this is great but it implies some restrictions on what political structure can reasonably work). If we have a unique economic policy, this will just end up simply being the policy of some dominating nation inside EU (and that's currently the case, more or less around a strong germany), and this ends up destabilizing politically all the others. I believe that a well functioning european union should be a federation of states that eases any wanted coordination and not a single federated state enforcing unwanted coordination (which benefits no one but the nation that happens to be in a de facto power position in the federation). A wannabe strong federated state would only exacerbate power positions, thus weakening the power of the group with regard to the outside world[#].
[#] This is a well-known social/evolution dynamic: group selection. Bees suicide when attacking: individually it's a weak behavior, but globally it makes the group more resilient. The balance between cooperation and competition is a balance between individual power and group power. Europe is a group of groups, so the balance is weirder but having too strong cooperation at the top level (eg enforcing a common economic policy) will weaken countries. But only some countries because of non-homogeneity, thus destabilizing everything by lowering local national resilience (to specific local concerns, which do exist). So i believe we have to accept that we need slightly stronger countries to make the european union stick together.