US telecoms costs are insane though. Paying less than $50 is basically impossible for a decent amount of data (5+GB). Google Fi for example is $70 + tax
In the UK, it's hard to spend more than £20/month (for sim only at least)
Mint [1], is $20/mo for unlimited data (throttled after 8GB), using the T-Mobile network. You get a discount for committing to a time period such as 1 year.
Visible [2] is $40/mo for "unlimited everything" (data, calls, text). This is Verizon's low-cost product on their own network.
Simple [3] is $40/mo for 15 GB, also using T-Mobile. You're throttled to 2G after that, though.
These are MVNOs, which are resellers that don't own their own networks. Traffic is deprioritized compared to the parent network's "native" customers. In practice, you might not notice at all, depending on how you use it. Deprioritization really only happens during congestion. During rush hour on the NYC subway might be a time you'd be affected, for example.
I don't think lower population density explains it fully, for example Australia has extremely reasonable phone plans - you can get 15GB data/unlimited talk/unlimited text for AUD$55/month (USD$37/month) [1], and it gets even cheaper if you get 12 months up front [2].
In the UK, it's hard to spend more than £20/month (for sim only at least)