This is one of the questions I did consider! The research I have done indicates that taxes in many countries are based on displacement rather than fuel mileage, as the dynos used today were not available when the legislation was written. From what I can ascertain, pushrods with larger displacements require more fuel to simply idle, but once on partial throttle, they can benefit from the broader torque curve and longer gearing by being under lighter loads which can mean better gas mileage. There has to be a break-even point where the amount of idling or very slow speed travel would hurt the additional displacement, but I don't believe I know what that point is or how to calculate it. It may also be a moot point considering start/stop systems and cylinder deactivation.
Furthermore, I think pushrods are not as popular in various non-V configurations (like inline engines) or forced induction engines (which reap greater benefits from more valves). I also think that in luxury cars, a DOHC setup creates smoother operations and its additional complexity can be something that a manufacturer sell as further proof of luxury, thus increasing the price.
Again, I'm happy to be corrected on any of these points (hence my user name), but the new C8R's dohc configuration has me scratching my head. I'm also trying to think big picture with my "unlimited class question.