€4.50/GB is the maximum what EU operators are allowed to charge each other for roaming data transit right now, and what an external operator like Twilio would probably have to pay them. Of course in each country you will find operators who offer better deals on their own networks - but Twilio can't reasonably build out their own worldwide mobile network.

Despite this, I can see this service being popular among companies who are willing to pay a bit more per GB to avoid negotiating with operators and getting SIM cards all over the world.