You might want to elaborate a bit on what you call "metering". A $50 surcharge for 50A vs 30A is basically just an extra-amenity fee, not really different than an extra charge for wifi or cable tv. You pay for having an enhanced capability (50A power) but it's still a fixed amount regardless of use. "Metering" is usually taken to mean charging by the kilowatt-hour (KWH) for actual power consumed, i.e. paying for what the electrical meter shows you used. There may or may not be a fixed fee in addition to the KWH rate, e.g. a "hookup fee" or "service fee" (which covers the power outlet itself, the meter, and the billing costs). Those things are typically separate because of the laws governing the sale of electricity in near-all states.
Most parks that offer both 30A and 50A service have an upcharge for 50A, either stated as such or simply rolled into different site rental rates. $3-$5 per day or $50-$100+ per month are fairly typical, but rates vary widely by location and owner whimsy. A fairly common practice is to rent monthly or annually without any power included in the lot rate and use electric meters to measure and bill for actual KWH consumed.