There is no generalization on this point. One has to price the proposed itinerary and act accordingly.
It may well be the case that a one-way ticket departing Europe for the US will approach or exceed the cost of a round-trip. But, it may not. In the US, almost all round-trips are priced as the sum of the one-ways.
A consideration here is to purchase the redemption as always and then purchase a round-trip departing Europe and schedule the return to Europe for a date when one might fly. If it comes to pass that one does want the trip, there is a one-way to get you there. If it does not, simply no show and you are none the worse off (if the price of the ticket exceeded the one-way).