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.