I have looked in more detail and note that the price of the Flexpreis Europa ticket seems to vary according to the date and time of travel, which is rather confusing as the conditions state that it is valid on any train on the booked date. Maybe someone else can clarify?
As an alternative there is a Normalpreis Europa ticket for €56.80, this is valid on any local train so ICs etc. in the Netherlands and RE/RB services in Germany - but NOT on ICEs. This appears to be the cheapest fully flexible option although at the expense of a slightly longer journey time.