Per country is cheaper per plan; regional is cheaper once you cross about three countries.
Measured across our own catalogue, a 5 GB / 30-day plan averages 28 USD for a single country and 38 USD for a regional plan. The regional premium is roughly 37 percent.
So two single-country plans (about 56 USD) already cost more than one regional plan. But you rarely need a full 5 GB in each of two countries — two 1 GB plans would be far cheaper than a 5 GB regional plan. The honest answer is to size the data first, then compare, rather than assuming regional is automatically better value.