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.
I'm in England. I get "unlimited" 3g through GiffGaff for £15 per month (that is the transitional price from an earlier £12 per month to the new £18 or £20 per month).
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.
But the EU price you quote is the legal maximum they can charge you without any package. So that $ 27.32 is the EU equivalent of the $ 750 that's in the post title. (And I think the US price also does not include VAT since, just like Europe, it varies by state)
It could very well be that they only charge by intervals of 30$/GB; meaning the minimum that they are willing to charge is 30$ but he could have used up to one gig.
