Cheapest to maintain it is pass a test once every two years (this is an alternative to the 12 hours). You probably won't pass the test after two years of not flying without two or three hours of lessons first. So maybe four hours every two years.
If you let it lapse beyond the two years then the bureaucratic procedures vary depending on various things, but the cost in all cases basically comes down to "training as necessary to pass a test", followed by the test, which is little different in principle to the within-two-years case but of course the longer you leave it the more retraining you'll need.