There wasn't a good, reasonable way to buy cheap roaming-data, at least not with my German contract. 150MB of roaming in the EU was 5€ with the purchase process (via SMS) being bugged and failing 50% of the time, sometimes charging my credit card instantly while delivering the "You can use your roaming data now" message days later when I was back home, customer support could not help because they "do not suggest that the data is available instantly". n=1, obviously.