Not even "barely more". With 30 seconds on Google I found a 6 acre private island in Northern Ontario with a fully furnished 2200 sq ft house connected to marine electric and phone service for 360k USD.
Something more remote is even cheaper, because there's a ton of supply of islands that are only accessible by float plane, and not much demand.
I mean, there's a decent supply of islands (many with houses) that go for barely more than a Bay Area single-family home.