Cost of living should fade in gradually as you cross a map. It mostly does not. Cross the right border and your daily budget can effectively reset, sometimes doubling, with nothing but a stamp in your passport to explain it.
The clearest example is Caribbean. Barbados prices out at 110 on our index (US = 100); Saint Lucia, 172km away, sits at just 52. The same pattern shows up on a land crossing backpackers make constantly: Costa Rica at 59 versus Nicaragua at 31, right next door. Or in the Baltics: Lithuania at 53 against Belarus at 26, a short drive apart.
None of these gaps are about distance. They are mostly about what the local economy is built to sell. Small islands with heavy resort tourism and everything shipped in, like Barbados or the Bahamas, price for the visitors who already expect to pay resort rates. Their less-touristed neighbors, without the same import costs or demand, simply do not. The same logic plays out with currency regimes and wage levels wherever one side of a border developed around tourism and the other did not.
For anyone stitching together a multi-country trip, the border can matter as much as the itinerary. Which side of a line you sleep on can swing your budget by half, and that is worth planning around, not discovering at the till.