Good question but its unrealistic to tie a consistent number for increase in NOI to submetering and make it work for all parks. This is a function of varying space rent between parks, whether people have POHs, etc… It is realistic to estimate the average usage per space. Knowing the water and sewer rates are in your area, you can calculate the expected increase in NOI increase in your park.
I can tell you that in my park we increased NOI north of 25%. Seems extreme, but those are our numbers. That won’t happen in every park but do the math for your park.