Note that the GP mistyped and it's actually 120MB for $30 (it's in one of the pictures of his text messages). Still not exactly reasonable, but at least not quite so outrageous.
You're specifically not allowed to tether using this plan and it's smart phones only - tablets, dongles, etc are not covered. I use over 10 GB of 3g data each month and they haven't complained.
So, 120 MB for $30 (£20) per month sounds pretty steep to me.
However roaming charges within the European Union are currently regulated to a maximum of €0.20 ($0.23) per MB. So a 120MB plan would be €24 ($27.32). That being said, if it isn't delayed by a future proposal, all roaming charges within the EU will be abolished on December 15th, 2015. True international roaming will still be ridiculously expensive, no doubt.
EDIT: My numbers don't include VAT (which is 20% in the UK) - so it's actually currently more expensive in the EU than in the example given in the post.
It's common enough for U.S. Carriers to do this.
No comment on whether that's a fair price.