I suspect operators will make their money back from "I forgot to turn off roaming" to "I took too much from my roaming data".
It goes to show why carriers lobbied so hard to prevent European citizens to buy subscriptions from a different country considering roaming would truly create European competition between carriers.
Keeping in mind Dutch,French and Belgian carriers are sometimes one and the same entity it shows liberalism and free market is only promoted and applied when it benefits the strong (carriers), not the weak (consumers).
It's still a very good and welcomed evolution for consumers though :).
Between Belgium France and the Netherlands, there isn't much of a problem. However, if you start comparing e.g. the Netherlands with Romania, things get weird. Those consumer markets are vastly different, Romania is much cheaper.
I'd expect that, due to differing expectations of infrastructure building a network in Romanio would also be cheaper.