Ignoring the argument on whether having everyone subsidising the few for some things is good or bad, does it even apply here?
Does "roaming" really cost anywhere near what they charge for it? Or does it only cost that because they could all charge each other that in hopes to make out with a little on top?
No, these days, in most cases, it costs them very little, and often nothing. If you're on a Vodafone or Telefonica network, then the network you're roaming on is probably also owned by Vodafone or Telefonica. Even where you have to roam on an unrelated network, the price difference has never made any sense.
Roaming costs what it costs to transport the (voice) data over the internet, because all of the telcos naturally now use that as their backbone and have been for 10+ years.
Does "roaming" really cost anywhere near what they charge for it? Or does it only cost that because they could all charge each other that in hopes to make out with a little on top?