In practice: I pay 20 € per month for unlimited mobile data. If I go to Italy and use my subscription there, I still pay 20 € per month for unlimited mobile data, but my operator has to pay 7 € per gigabyte to the Italian roaming partner.
Because there are more people traveling from north EU to south EU than vice versa, and competition of mobile operators works much better in northern EU, this is practically a monetary transfer to support the economies of southern Europe. And then there's the possibility that an Italian person acquires a SIM from my operator, because local Italian ones don't sell subscriptions with unlimited mobile data...
My operator has to counter this by a price hike and/or disabling of roaming.
Most countries will have 3-4 operators so that there is competition and I doubt that €7/GB is the cheapest an Italian operator can offer to a wholesale client.
Has that changed?
(tho they plan to lower the regulated rates over time)
So, e.g. Telia doesn't have to pay any roaming charge to WIND Hellas if a Telia customer from Sweden goes to Greece and connects that network?
Previously I heard a cap of 7 € per gigabyte for data fees was regulated.
If I divide my monthly internet bill with the amount of data I transfer during it, then price of GB of data transfer is indeed close to nothing.
But even the most price-gouging "captive audience" transit providers (e.g. in data centres without multiple choices of carriers) rarely charge more than ~$10/Mbps (depending on utilisation down to ~$40 per TB transferred or about 0.04 cents per GB...)
The EU parliament wanted to go much further, starting at 4 and gradually reducing to 1. They'll get pushed further down eventually, but at the same time nobody wants to overdo this and cause problems for these providers by going too fast.
Now those operators that have been competitive will suffer. Those that have been extractive towards their own customers and their competitors will benefit.