I'm not a huge fan of transit "efficiency", if you even want to call it that, and I think that the healthiest rate of cost recovery is 55-65% tops. In that range, clearly enough people are hopping on board to justify the current level of service, but government subsidies are high enough that the transit authority can still divert money into system expansion.
Some of the toughest years for the TTC occurred during the late 90s and early 2000s - ridership was plummeting, service was being cut back, and the system was in decline. During those years however, "efficiency" was at a modern day high, comfortably above 80%. The last thing I would want is to ever go back to those days of high cost recovery.